A noticeable size difference between your dog’s testicles usually signals one of a handful of conditions: a tumor, an infection, a hernia, or, less commonly, a twist in the spermatic cord. Some of these are slow-developing and treatable with routine surgery, while others need urgent attention within hours. The cause matters, so understanding what to look for can help you act at the right pace.
Testicular Tumors Are the Most Common Cause
In intact (unneutered) male dogs, testicular tumors are one of the most frequent reasons one testicle grows noticeably larger. Three tumor types account for nearly all cases: Sertoli cell tumors, seminomas, and interstitial cell tumors (also called Leydig cell tumors). All three can cause a firm, irregular mass inside the testicle, and on ultrasound they range from about 4 to 55 mm in diameter. Because most testicular tumors grow slowly, you may not notice the size change until the affected testicle is clearly larger than the other one.
The good news is that most canine testicular tumors stay localized and don’t spread to other organs. Interstitial cell tumors, the most common type, carry the lowest risk of spreading. Sertoli cell tumors and seminomas have a somewhat higher potential for malignant behavior, especially when the affected testicle never fully descended into the scrotum (a condition called cryptorchidism). Surgical removal of the affected testicle is curative in the vast majority of cases.
Age is the biggest risk factor. Testicular tumors become increasingly common as intact dogs get older, particularly after age 7 or 8. Breed also plays a role, with some breeds showing higher rates than others.
Cryptorchidism Raises the Stakes
If your dog has one testicle that never fully dropped into the scrotum, it may be retained in the abdomen or inguinal canal. A retained testicle is significantly more likely to develop a tumor than one sitting normally in the scrotum. About 70% of Sertoli cell tumors that form in abdominal testes are hormonally active, meaning they produce excess estrogen. This can trigger a set of visible changes sometimes called feminizing syndrome: symmetrical hair loss on both sides of the body, darkened skin, swollen nipples, a puffy penile sheath, and behavioral shifts like other male dogs showing sexual interest in your dog. If you notice any combination of these signs alongside testicular asymmetry, a retained testicle with a hormone-producing tumor is a strong possibility. Early neutering of cryptorchid dogs is widely recommended specifically to prevent these tumors.
Infection and Inflammation
Orchitis (inflammation of the testicle) and epididymitis (inflammation of the tube that stores sperm next to the testicle) frequently occur together and can make one side of the scrotum noticeably swollen. The key difference from a tumor is speed and pain. Infections tend to come on faster, and the swollen testicle is usually warm and painful to the touch. Your dog may lick the area excessively, walk stiffly, or resist being handled around the hindquarters. The scrotal skin itself may become red, swollen, or raw.
Bacteria are the usual culprits. They can reach the testicle through the bloodstream, travel up from a urinary tract or prostate infection, or enter through a bite wound or other puncture. Fungal and viral causes are less common but possible. Epididymitis can be unilateral or bilateral, and in some chronic cases the inflammation is subtle enough to go unnoticed until a vet finds it on ultrasound. Bacterial infections from the bladder or prostate can spread to the epididymis and testicle, so dogs with a history of urinary problems deserve closer monitoring.
Testicular Torsion Is an Emergency
Testicular torsion occurs when the spermatic cord twists, cutting off blood flow to the testicle. It is rare in dogs compared to the other causes on this list, but it is the one you need to recognize fast. The twist can range from a loose single rotation to several tight revolutions, and the tighter and longer-lasting it is, the worse the outcome.
Signs come on suddenly: acute pain, rapid swelling of one side of the scrotum, and a testicle that may sit higher than normal or at an unusual angle. Your dog may vomit or refuse to move. Research on torsion (primarily in human medicine, where the condition is better studied) shows that salvage rates exceed 90% when surgery happens within 6 hours. After 12 hours, the chance of saving the testicle drops to about 50%. Beyond 24 hours, it falls below 10%, with tissue death setting in around 8 to 10 hours. The timeline in dogs is comparable. If your dog suddenly yelps, the scrotum swells rapidly on one side, and he seems to be in serious pain, treat it as an emergency and get to a vet immediately.
Inguinal Hernia
Sometimes what looks like a swollen testicle is actually a loop of intestine that has slipped through the inguinal canal into the scrotum. This is called an inguinal or scrotal hernia. The scrotum appears enlarged on the affected side, and on careful palpation a vet may feel something that doesn’t have the firm, round shape of a testicle. If the trapped intestine loses its blood supply, the area becomes extremely painful and the situation turns into a surgical emergency. In milder cases, the hernia may come and go, with the scrotum looking larger at some times than others. Ultrasound is the best way to confirm it, since the vet can actually see the intestinal loops moving inside the scrotum.
What the Vet Visit Looks Like
Your vet will start with a physical exam, feeling both testicles for size, shape, firmness, and pain. A soft, painful, warm swelling points toward infection. A firm, irregular, painless lump is more suggestive of a tumor. From there, scrotal ultrasound is the standard next step. It can distinguish between solid masses, fluid buildup, herniated tissue, and signs of compromised blood flow (as in torsion). If a mass is found, fine-needle aspiration, where a small needle draws out a few cells for microscopic analysis, can help identify the type of growth before surgery.
Blood work may also be part of the workup, particularly if your vet suspects a hormone-producing tumor or a systemic infection. Elevated estrogen levels, for instance, support a diagnosis of a functioning Sertoli cell tumor.
Treatment and Recovery
For tumors, surgical removal of the affected testicle (or both, in a full neuter) is the standard treatment and is curative for the vast majority of dogs. Most testicular tumors have not spread by the time they’re discovered, so the prognosis after surgery is excellent. Recovery from the procedure typically takes 10 to 14 days, with restricted activity and an e-collar to prevent licking at the incision site.
For infections, treatment depends on severity. Mild cases may respond to a course of antibiotics, but if the infection is severe or has caused permanent damage to the testicle, surgical removal may still be the best option. Chronic infections that go untreated can lead to testicular atrophy, where the testicle shrinks and loses function over time.
For torsion or an incarcerated hernia, emergency surgery is the only option, and the sooner it happens the better the outcome. In all of these scenarios, neutering an intact dog with a testicular problem not only treats the current issue but eliminates the risk of future problems on the remaining side.