Swollen lymph nodes in the groin can be caused by several STIs — most commonly genital herpes and syphilis, plus the now-rare chancroid and early HIV. The key clue is the node's quality: tender, painful nodes point toward herpes or chancroid, while firm, painless ones suggest syphilis. Many causes overlap, so testing is the only way to be sure.
painful blisters that crust over; tends to recur
painful, soft, ragged ulcer(s)
a single painless sore (chancre); later a body rash
flu-like illness weeks after exposure, then silent
| Item | Value |
|---|---|
| Genital herpes | managed — painful blisters that crust over; tends to recur |
| Chancroid | curable — painful, soft, ragged ulcer(s) |
| Syphilis | curable — a single painless sore (chancre); later a body rash |
| HIV | managed — flu-like illness weeks after exposure, then silent |
Your groin (inguinal) lymph nodes are filtering stations for the genitals, lower belly, and legs. When an infection sets up nearby, those nodes swell as your immune system goes to work. A sore on your penis, vulva, or anus can show up as a tender lump in the crease of your thigh. Below I'll walk through which STIs do this, how they differ, and when something is not an STI at all.
Which STIs cause swollen lymph nodes in the groin
Genital herpes
Genital herpes is caused by two viruses, HSV-1 and HSV-2 CDC, About Genital Herpes. A first outbreak is the most likely time for swollen glands: blisters break into painful sores that take a week or more to heal, often alongside flu-like symptoms — fever, body aches, and swollen lymph nodes in the groin. The nodes here tend to be tender. Repeat outbreaks are shorter and milder, sometimes warned by a tingling or burning prodrome.
Most people with herpes have no or very mild symptoms and never know they carry it; the majority of HSV-2 infections are undiagnosed. A tender groin node with a cluster of painful sores fits herpes, but the absence of obvious sores doesn't rule it out. Once you have herpes you keep it, though outbreaks usually get less frequent over time; some people explore alternative herpes treatments to manage flare-ups alongside standard antivirals.
Chancroid
Chancroid is a bacterial STI caused by Haemophilus ducreyi, and it's become rare in the United States CDC, Chancroid Tx Guidelines 2021. Its signature is one or more deep, painful genital ulcers together with tender, pus-filled (suppurative) lymph nodes in the groin that can soften and drain. The painful ulcer separates it from syphilis, whose sore is painless. If you have a raw, painful ulcer with a sore lump beside it, read more about chancroid symptoms and get checked, because diagnosis hinges on ruling out herpes and syphilis first.
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with the right antibiotics CDC, About Syphilis. In the primary stage you get one or more painless, firm, round sores (chancres) at the site of infection, appearing about three weeks after exposure (incubation runs from about ten to ninety days). The groin nodes that swell with primary syphilis are classically firm and painless, the opposite feel of herpes or chancroid.
The chancre lasts a few weeks and heals on its own whether or not you treat it, which fools people into thinking the problem is gone. The secondary stage can follow with a rough red or reddish-brown rash — often on the palms and soles — plus fever, sore throat, patchy hair loss, fatigue, and more widespread swollen lymph nodes. Anyone pregnant should know syphilis crosses to the baby, so syphilis in pregnancy is screened and treated promptly.
HIV (early infection)
HIV is a virus that attacks the immune system CDC, About HIV. Within two to four weeks of infection, many people develop a flu-like illness called acute retroviral syndrome: fever, chills, rash, night sweats, muscle aches, sore throat, fatigue, mouth ulcers, and swollen lymph nodes hiv.gov, HIV Symptoms. With HIV the swelling isn't limited to the groin — nodes in the neck and armpits often swell too, reflecting a body-wide immune response rather than a single local sore.
Some people feel nothing at all during this phase, and the symptoms are easily mistaken for other viral illnesses. After the acute phase, HIV can stay silent for years (clinical latency). Starting therapy early protects your health and, because it suppresses the virus, helps prevent passing it on; this is why earlier hiv treatment can help prevention matters so much.
When it's NOT an STI
Not every swollen groin node is sexual. The inguinal nodes drain the whole lower limb and lower abdomen, so an ordinary cause is common:
- A skin infection on the leg, foot, or genitals — a cut, ingrown hair, boil, or cellulitis (a spreading bacterial skin infection) — can swell the nearby nodes as your body fights it.
- An athlete's foot or other fungal infection on the foot can quietly enlarge a groin node on the same side.
- A recent injury, insect bite, or scrape below the waist can do the same.
- Reactive swelling from any nearby inflammation, even without an obvious wound.
The tender-versus-firm quality of the node helps here too. A hot, tender, fast-growing node usually means active local infection, while a single firm painless node deserves an STI workup.
How to tell them apart
Three features do most of the sorting: whether the node is tender or firm, whether there's a sore and whether that sore hurts, and whether you also have body-wide symptoms. Painful sore plus tender node leans herpes or chancroid; painless firm sore plus painless node leans syphilis; flu-like illness with swelling in several areas and no single sore raises HIV. These overlap too much to call by sight alone, and several are frequently silent. A test is what settles which one (if any) it is.
Side-by-side comparison
| Cause | Node feel | Sore / ulcer | Other clues |
|---|---|---|---|
| Genital herpes | Tender | Painful blisters that break into sores | Flu-like symptoms in a first outbreak; often very mild or silent |
| Chancroid | Tender, pus-filled, may drain | Deep, painful ulcer(s) | Rare in the U.S.; diagnosed after herpes and syphilis are excluded |
| Syphilis (primary) | Firm, painless | Painless, firm, round chancre that heals on its own | Sore appears about 3 weeks after exposure; later rash on palms/soles |
| Early HIV | Swollen, often in several areas | Usually none (may have mouth ulcers) | Flu-like illness 2–4 weeks after exposure; fever, rash, night sweats |
| Non-STI (skin/leg infection) | Hot, tender, fast-growing | A visible cut, boil, or rash below the waist | Often one-sided; tracks to a wound or athlete's foot |
How it's tested
Testing depends on what's suspected: a swab of any sore for herpes (confirmed by type-specific NAAT or culture) CDC, Herpes Testing, two blood tests for syphilis — a nontreponemal test like RPR plus a treponemal test CDC Syphilis Lab Recs, 2024 — and a blood test for HIV. In practice it's a urine sample, a self-collected swab, or a quick exam, with results usually back in a few days. You can get tested at a clinic or order a panel; just make sure you're past the right window for each infection — here's when to test after exposure.
What to do next
If you have a sore, a swollen node, or a recent exposure that worries you, get tested rather than guessing, because the overlapping symptoms make self-diagnosis unreliable. Most of these infections are treatable: syphilis and chancroid are curable with antibiotics, while herpes and HIV are managed long-term with medication. Don't squeeze or drain a swollen node yourself. Testing is free or low-cost at health departments, Planned Parenthood, and Title X clinics.
Red flags — when to get seen urgently
- The node is hot, rapidly enlarging, or the overlying skin is red and spreading — a possible deep skin infection.
- You have a high fever, chills, or feel unwell along with the swelling.
- A node is starting to soften and drain pus.
- A genital sore is large, very painful, or not healing.
- You're pregnant and have any new sore or swelling — get checked the same day, because syphilis affects the baby.
- Severe pain in the groin or testicle, which needs same-day evaluation.