An enlarged testicle usually results from fluid buildup, swollen veins, infection, or a hernia pushing into the scrotum. Less commonly, it can signal testicular cancer or a surgical emergency called torsion. Most causes are benign and treatable, but because a few are serious or time-sensitive, any new or sudden swelling deserves a prompt evaluation.
Hydrocele: Fluid Around the Testicle
A hydrocele is one of the most common reasons a testicle looks or feels larger than normal. It happens when extra fluid collects in the thin sac that surrounds the testicle. The enlargement is typically painless, develops slowly, and may fluctuate slightly in size throughout the day. Hydroceles are especially common in newborns, where a small channel between the abdomen and scrotum hasn’t fully closed yet, allowing fluid to trickle down. In adults, fluid can accumulate after an infection, injury, or inflammation, or sometimes for no identifiable reason.
Because a hydrocele is just fluid, the swelling usually feels smooth and uniform rather than lumpy. Holding a small flashlight against the scrotum in a dark room will often cause the swelling to glow (a trick called transillumination), which helps distinguish fluid from a solid mass. Small hydroceles that cause no discomfort are often left alone. Larger ones can be drained or surgically corrected if they become heavy or uncomfortable.
Varicocele: Swollen Veins in the Scrotum
A varicocele is a cluster of dilated veins inside the scrotum, similar to varicose veins in the leg. It affects up to 15% of men and shows up most often on the left side. The reason for this left-side preference is anatomical: the left vein draining the testicle empties into the kidney vein at a sharp angle and can be compressed between two major blood vessels (the aorta and an overlying artery). That compression raises pressure in the vein, forcing blood to pool and the vein to stretch.
Varicoceles often feel like a “bag of worms” above or behind the testicle and tend to become more noticeable when you stand or bear down. Many men have no symptoms at all and discover a varicocele during a routine exam. Although varicoceles are sometimes linked to reduced fertility, most men with one have no trouble conceiving. Treatment is only considered when the varicocele causes persistent pain or when fertility testing shows a problem.
Epididymitis: Infection or Inflammation
The epididymis is a coiled tube sitting behind each testicle where sperm mature before ejaculation. When it becomes infected or inflamed, the affected side swells, often quickly, and the pain can radiate into the lower abdomen or groin. The scrotum may feel warm and look red.
In sexually active men under 35, the most common culprits are bacteria transmitted through sex, particularly chlamydia and gonorrhea. In older men, the infection more often comes from urinary tract bacteria, especially when an enlarged prostate or other blockage makes it harder to fully empty the bladder. Antibiotics clear most cases within a few weeks, though the swelling itself can take longer to resolve completely. If the infection spreads from the epididymis into the testicle itself, the condition is called epididymo-orchitis, and the swelling tends to be more pronounced.
Inguinal Hernia
An inguinal hernia occurs when a section of intestine or abdominal lining pushes through a weak spot in the groin wall. In men, that weak spot is the inguinal canal, the same passageway the spermatic cord travels through to reach the scrotum. A large hernia can follow that path all the way down, making one side of the scrotum visibly bigger.
Unlike most other causes of scrotal swelling, a hernia tends to change with position and activity. You might notice the bulge grows when you stand, cough, or lift something heavy, then shrinks or disappears when you lie down. It often produces a burning or aching sensation at the bulge site, along with pressure in the groin. If a hernia becomes trapped (incarcerated) and its blood supply gets cut off, it becomes a surgical emergency with sudden, intense pain and nausea.
Spermatocele: A Cyst Near the Epididymis
A spermatocele is a fluid-filled cyst that forms in the epididymis, usually at the top or behind the testicle. These cysts are noncancerous and typically don’t affect fertility or require treatment. Many men never know they have one until a doctor finds it during an exam or an imaging study done for another reason.
A spermatocele may stay small indefinitely. If it grows large enough, you might notice a sense of heaviness or mild discomfort in the affected testicle. Surgical removal is an option for cysts that cause bothersome symptoms, though surgery carries a small risk of damaging the sperm-carrying structures nearby.
Testicular Cancer
Testicular cancer is relatively rare, but it’s the most common cancer in young men. Over half of new cases are diagnosed between the ages of 20 and 34. The classic presentation is a painless, hard lump on or within the testicle, which can make the entire testicle feel larger or heavier than the other side. Some men also notice a dull ache in the lower abdomen or groin.
Not every lump is cancer. Cysts, calcifications, and benign growths can feel similar. The key distinguishing step is a scrotal ultrasound, which can differentiate a solid mass from a fluid-filled cyst and identify features suggestive of different tumor types. When caught early, testicular cancer has an excellent cure rate, which is why noticing a change in size or texture matters.
Testicular Torsion: A Time-Sensitive Emergency
Torsion happens when the testicle twists on its spermatic cord, cutting off its own blood supply. The hallmark is sudden, severe pain on one side, often accompanied by nausea or vomiting. The affected testicle may sit higher than usual or at an odd angle, and the scrotum swells rapidly.
This is a true emergency. Animal studies show that after about six hours of complete blood flow loss, the sperm-producing cells in the testicle begin to die, and by ten hours of significant twisting, most patients will have permanent damage. Surgeons treat torsion by manually or surgically untwisting the cord and stitching the testicle in place to prevent recurrence. The diagnosis is made on physical exam alone when suspicion is high; waiting for imaging can waste critical time. Torsion is most common in adolescents but can happen at any age, and it is frequently misdiagnosed as epididymitis in older patients.
How Doctors Evaluate the Swelling
A physical exam gives your doctor a lot of information: whether the swelling is tender or painless, whether it feels like fluid or a solid mass, and whether it changes when you stand or lie down. For most cases, the next step is a scrotal ultrasound. This painless imaging test uses sound waves to create a detailed picture of the testicle and surrounding structures. It can show whether a mass is solid or cystic, identify dilated veins, measure fluid collections, and assess blood flow to rule out torsion. Urine tests or swabs may be added if infection is suspected.
Monthly Self-Checks
Starting around age 15, it’s a good habit to examine your testicles once a month. The best time is during or right after a warm shower, when the scrotal skin is relaxed. Stand up, lift the penis out of the way, and gently roll each testicle between your thumb and fingers. You’re feeling for any hard lumps, changes in size, or areas of unusual firmness. The epididymis at the top-back of each testicle is naturally soft and slightly tender, so don’t mistake it for an abnormality. Because it’s uncommon to develop a problem on both sides simultaneously, comparing one testicle to the other is a useful reference point. Any new lump, swelling, or persistent heaviness that wasn’t there before is worth getting checked.