Swollen testicles have several possible causes, ranging from infections that clear up with antibiotics to emergencies that need surgery within hours. The most common cause in adults is epididymitis, an infection of the coiled tube behind each testicle, which accounts for over 600,000 emergency department visits per year in the United States alone. But other conditions, including torsion, hernias, fluid buildup, and cancer, can also cause swelling.
Epididymitis: The Most Common Cause
Epididymitis is inflammation of the epididymis, the small tube that stores and carries sperm from the back of each testicle. In men between 18 and 50, it’s responsible for roughly 1 in every 144 outpatient visits. The swelling usually develops over a day or two rather than all at once, and it’s often accompanied by warmth, tenderness on one side, and sometimes painful urination.
In sexually active men under 35, the infection is most often caused by the same bacteria behind chlamydia or gonorrhea. In older men or men who are the insertive partner during anal sex, gut bacteria like E. coli are the more typical culprit. Either way, the infection is treatable with antibiotics. When the infection spreads from the epididymis into the testicle itself, it’s called epididymo-orchitis, which produces more diffuse swelling and can take longer to resolve.
Testicular Torsion: A Time-Sensitive Emergency
Testicular torsion happens when a testicle rotates on the spermatic cord, the structure that delivers blood to it. That twist chokes off blood flow, causing sudden, severe pain and rapid swelling. You may also notice nausea, vomiting, abdominal pain, or that one testicle sits higher than usual or at an odd angle. The pain typically hits without warning and escalates fast.
This is the one cause of testicular swelling you cannot afford to wait on. A systematic review of over 1,200 patients found that the testicle can be saved 97% of the time if surgery happens within the first six hours. Between 7 and 12 hours, that drops to about 79%. By 13 to 24 hours, it falls to 54%. Beyond 24 hours, only about 18% of testicles survive. If you have sudden, intense scrotal pain with swelling, treat it as an emergency.
Torsion is most common in adolescents and young men, with an estimated incidence of 1 in 4,000 males per year. Roughly 1 in 160 men will experience it before age 25. Newborns can also develop a form of torsion, though the mechanism differs slightly.
Hydrocele and Varicocele
Not all scrotal swelling involves the testicle itself. A hydrocele is a buildup of fluid in the thin sac surrounding the testicle. It creates a smooth, painless swelling that can make one side of the scrotum look noticeably larger. Hydroceles are common in newborns and often resolve on their own within the first year. In adults, they can develop after injury, infection, or for no obvious reason. They’re generally harmless but worth getting checked if the swelling is new or growing.
A varicocele is a cluster of enlarged veins in the scrotum, similar to varicose veins in the legs. Blood pools in the veins instead of circulating efficiently, creating a soft, lumpy swelling that’s sometimes described as feeling like a “bag of worms.” Varicoceles almost always appear on the left side. The telltale pattern is a dull, aching discomfort that worsens throughout the day or after standing for long periods and improves when you lie down. The affected testicle may eventually become noticeably smaller than the other one. Varicoceles can affect fertility, so they’re worth mentioning to your doctor even if they aren’t particularly painful.
Inguinal Hernia
An inguinal hernia occurs when part of the intestine or abdominal lining pushes through a weak spot in the lower abdominal wall. In men, a large hernia can extend down into the scrotum, causing swelling that may look and feel like a swollen testicle but is actually displaced bowel tissue. The swelling often increases when you stand, cough, or strain, and it may shrink or disappear when you lie down.
Most inguinal hernias can be gently pushed back in. If the hernia becomes trapped (incarcerated) or its blood supply gets cut off (strangulated), you’ll notice sudden worsening pain, nausea, vomiting, and the bulge may turn red or purple. A strangulated hernia is a surgical emergency.
Testicular Cancer
The first sign of testicular cancer is often a painless lump or swelling in one testicle. Unlike infections or torsion, the swelling typically develops gradually and may not hurt at all. You might notice a feeling of heaviness in the scrotum, a dull ache in the lower abdomen or groin, or a change in how the testicle feels compared to the other side. Some men do experience discomfort, but the classic presentation is a firm, painless mass.
Testicular cancer is relatively rare compared to infections and torsion, but it’s the most common cancer in men aged 15 to 35. The good news is that it has one of the highest cure rates of any cancer, especially when caught early. Any new, hard lump that doesn’t go away within a week or two should be evaluated by a doctor.
How Testicular Swelling Gets Diagnosed
A scrotal ultrasound is the gold standard for evaluating swelling. It’s painless, fast, and noninvasive. The imaging combines high-resolution pictures with Doppler blood flow measurement, which lets the technician see whether blood is reaching the testicle normally (ruling out torsion), whether the swelling is solid or fluid-filled (distinguishing tumors from hydroceles), and whether the epididymis is inflamed. For detecting tumors, ultrasound has about 85% sensitivity and 99% specificity, meaning it’s very good at confirming cancer when it’s present and excellent at ruling it out when it isn’t.
Your doctor will also ask about how quickly the swelling started, whether you have pain, any recent sexual activity, urinary symptoms, and whether you’ve had trauma to the area. The timeline matters: sudden onset over minutes suggests torsion, gradual onset over days points toward infection, and slow, painless growth over weeks raises concern for a tumor or hydrocele.
How to Do a Self-Exam
Regular self-exams help you notice changes early. Stand unclothed in front of a mirror, ideally after a warm shower when the scrotal skin is relaxed. Hold your penis out of the way and visually check for any obvious swelling or asymmetry. Then examine each testicle individually: place your index and middle fingers underneath and your thumbs on top, and gently roll the testicle between them. You’re feeling for hard lumps, smooth bumps, or any change in size, shape, or firmness compared to what’s normal for you.
It’s normal for one testicle to be slightly larger than the other or to hang a bit lower. The epididymis, the soft ridge along the back of each testicle, can feel bumpy and is not a cause for concern. What you’re looking for is something new: a firm area that wasn’t there before, a noticeable size increase, or a spot that feels distinctly different from the surrounding tissue.