Having one testicle slightly smaller than the other is completely normal. Most men have some degree of asymmetry, and a size difference of up to 10% between the two sides falls well within the expected range for adults. The testicles also typically hang at different heights, which can make the size difference more noticeable. That said, a sudden or significant change in size can sometimes signal a condition worth checking out.
Why Some Asymmetry Is Normal
Adult testicles range from about 10 to 21 milliliters in volume, roughly the size of a small plum. That’s a wide range even for a single person’s two testicles, and a mild mismatch between left and right is the rule rather than the exception. Studies of testicular measurements show that even a 1-millimeter difference in width translates to about a 10% volume difference in adults. In other words, a variation you couldn’t spot with your eyes can show up as a measurable size gap on an ultrasound.
The two testicles also hang at different heights. Each one continuously adjusts its position using a muscle that contracts and relaxes to regulate temperature. According to anatomist Stany Lobo, each testicle migrates in its own independent orbit to maximize the scrotal surface area available for cooling. This natural movement means the positioning and apparent size can shift throughout the day.
Varicocele: The Most Common Cause
A varicocele is an enlargement of veins inside the scrotum, similar to a varicose vein in the leg. It’s the most frequent medical reason one testicle becomes noticeably smaller than the other. Varicoceles occur overwhelmingly on the left side due to differences in how blood drains from each testicle. Over time, the pooling of warm blood can cause the affected testicle to shrink.
In a study of 432 men evaluated for infertility, about half had some form of varicocele, with left-sided varicoceles being the most common. Men with left-sided varicoceles were twice as likely to have a measurable size discrepancy between their testicles compared to men without one (26% versus 13%). That size gap mattered: men with varicoceles and a volume discrepancy had significantly lower sperm counts, reduced sperm motility, and lower total motile sperm counts than those with varicoceles but evenly sized testicles. Importantly, when the same analysis was done on men without varicoceles, the size difference alone didn’t correlate with worse semen quality.
Varicoceles often feel like a “bag of worms” above the testicle when you’re standing. Many cause no symptoms at all and are discovered during a routine exam or fertility evaluation. In adolescents, surgical repair has been shown to allow the smaller testicle to undergo catch-up growth, which is why early diagnosis can matter.
Fluid Collections: Hydroceles and Spermatoceles
Sometimes the size difference isn’t about the testicle itself but about fluid surrounding it. A hydrocele is a collection of fluid that wraps around the front and sides of the testicle, making that side of the scrotum look swollen or larger. It’s painless and often develops gradually. A spermatocele, by contrast, is a small fluid-filled cyst that forms near the top of the testicle. Most spermatoceles are under 1 centimeter and go unnoticed, though they can occasionally grow to several centimeters.
Both conditions are benign. The key distinction is that neither one changes the testicle itself. If the testicle feels normal when you examine it but one side of the scrotum seems puffier, fluid is the likely explanation. An ultrasound can confirm this quickly.
Testicular Atrophy From Hormones or Medications
The testicles can actually shrink over time in response to hormonal changes, a process called atrophy. When the body receives testosterone from an outside source, such as testosterone replacement therapy or anabolic steroids, it signals the brain to stop telling the testicles to produce their own. Without that signal, the testicles gradually decrease in size. The shrinkage can affect both sides but sometimes appears uneven, making one noticeably smaller.
Estrogen-based medications and certain other drugs can trigger a similar process. The lost volume reflects a real reduction in sperm-producing and hormone-producing cells, which can lower both sperm count and natural testosterone output.
When Size Changes Could Signal Cancer
The most common first sign of testicular cancer is a lump on the testicle or one testicle becoming swollen or larger. Most testicular tumors are painless, which is why a new size difference that develops over weeks or months deserves attention even if nothing hurts. A hard lump, a change in shape, or a feeling of heaviness in the scrotum are the classic warning signs.
Non-cancerous conditions can mimic these symptoms. Inflammation of the testicle (orchitis) or of the coiled tube behind it (epididymitis) can cause swelling and pain that make one side appear bigger. These infections typically come on gradually and may accompany urinary symptoms or fever. The important distinction is that cancer tends to present as a firm, localized mass you can feel within the testicle, while infection produces more diffuse tenderness and swelling.
Sudden Pain Is a Different Situation
A sudden, severe pain in one testicle is a separate concern from a size difference. Testicular torsion, where the testicle twists on its blood supply, causes acute pain that comes on fast. The affected testicle often rides higher than normal and sits in an unusual orientation. Lifting the testicle typically makes the pain worse. This is a surgical emergency that requires treatment within hours to save the testicle.
Epididymitis, by comparison, develops gradually. Pain starts behind the testicle and may radiate to the lower abdomen. Lifting the testicle sometimes provides relief, a clinical finding known as the Prehn sign. Fever and urinary symptoms are common. Both conditions can cause one side to swell and look larger, but the speed of onset is the critical difference: minutes for torsion, hours to days for infection.
How to Check Yourself
A self-exam is straightforward. The easiest time is during or after a warm shower, when the scrotal skin is relaxed. Stand in front of a mirror and hold your penis out of the way so you can see both sides of the scrotum. Look for any visible swelling or asymmetry that’s new to you.
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 rounded bumps, or any change in size, shape, or firmness compared to what you’re used to. The epididymis, a soft ridge along the back of each testicle, is normal and shouldn’t be confused with a lump. If you find something new or unusual, that’s worth a visit to your doctor, who will likely order an ultrasound to get a clear picture of what’s going on.
Does Size Asymmetry Affect Fertility?
For most men, a minor difference in testicle size has no impact on fertility or testosterone levels. Average testosterone levels are similar between men with and without size discrepancies when no underlying condition is present. The concern arises specifically when the size difference is caused by something, like a varicocele, that damages testicular function over time.
In men with significant left-sided varicoceles, those with a measurable volume discrepancy had an average total motile sperm count of about 26 million, compared to 66 million in men with varicoceles but no size gap. Sperm motility dropped from roughly 41% to 28%. These differences are clinically meaningful for someone trying to conceive. But in men with unexplained infertility and no varicocele, the same size discrepancy showed no relationship to semen quality at all. The takeaway: the size difference itself isn’t the problem. The underlying cause is what matters.