If your dog appears to have only one testicle, the missing one almost certainly didn’t fail to develop. It’s still inside his body, stuck somewhere along the path it was supposed to travel during early development. This condition is called cryptorchidism, and it’s the most common reproductive abnormality in male dogs. In a large study of over 3,500 castrated dogs, roughly 7% were cryptorchid.
How Testicles Normally Descend
In a developing puppy, both testicles start near the kidneys inside the abdomen. A small cord of tissue gradually shortens as the puppy grows, pulling each testicle down through the abdomen, through a narrow canal in the groin, and into the scrotum. This process usually finishes by six to eight weeks of age. If one or both testicles haven’t arrived in the scrotum by that point, the puppy is considered cryptorchid.
The retained testicle can get stuck in one of two places: inside the abdomen or in the inguinal canal (the passageway through the groin). Sometimes you can feel a small lump in the groin area where the testicle is lodged. Other times, especially when it’s deep in the abdomen, there’s nothing to feel from the outside at all.
What Causes It
Cryptorchidism has a strong genetic component. Certain breeds are significantly more prone to it, with German Shepherds, Boxers, and Chihuahuas being notably overrepresented in studies. The condition runs in families, which is why veterinary organizations and breed clubs generally recommend against breeding cryptorchid dogs. If your dog passes on the trait, his offspring face the same risks.
Beyond genetics, the cause involves a mix of anatomical and developmental factors. The cord that pulls the testicle into position may not shorten properly, or the inguinal canal may be too narrow. In some cases, hormonal signaling during fetal development doesn’t work as expected.
Why It’s a Health Concern
A retained testicle isn’t just a cosmetic issue. The body’s internal temperature is several degrees warmer than the scrotum, and testicles weren’t designed to function in that heat long-term. This creates two serious risks.
The biggest concern is cancer. Cryptorchid dogs have roughly 9 to 14 times the risk of developing testicular cancer compared to dogs whose testicles descended normally. Some research puts specific tumor types even higher: one study found a 23 times greater risk for one type of testicular tumor and a 16 times greater risk for another. These aren’t rare odds worth ignoring.
The second risk is torsion, where the retained testicle twists on its blood supply inside the abdomen. This is extremely painful and constitutes a veterinary emergency. Because you can’t see or easily monitor an internal testicle, torsion or tumor growth can progress before you notice any outward signs.
A retained testicle also typically can’t produce viable sperm due to the elevated body temperature, though it does continue producing hormones. Your dog will still display male behaviors driven by testosterone.
How It’s Diagnosed
Your vet will start with a physical exam, feeling along the groin and scrotum. If the testicle is in the inguinal canal, it can often be located by touch. When the testicle is deeper in the abdomen and can’t be felt, an ultrasound can help pinpoint its location. In some cases, particularly with dogs adopted as adults where surgical history is unclear, a hormone test can confirm whether testicular tissue is still present in the body.
Surgery Is the Standard Treatment
The standard recommendation is to surgically remove the retained testicle, and most vets will neuter the dog entirely at the same time. This eliminates the cancer risk and prevents the dog from passing the trait to future generations.
The complexity of surgery depends on where the testicle is stuck. If it’s in the inguinal canal, the procedure is relatively straightforward, similar to a standard neuter with an additional small incision. If the testicle is inside the abdomen, the surgery is more involved. Your vet will make an abdominal incision to locate and remove it. Research from the Journal of the American Veterinary Medical Association found that the specific type of abdominal incision matters for complication rates, so this is a procedure best handled by an experienced surgeon.
Most dogs do well after surgery. Recovery follows the same general timeline as a standard neuter: about 7 to 10 days of restricted activity. During that period, you’ll want to keep your dog from running, jumping, or playing roughly, as movement can disrupt healing or cause the incision to reopen. Leash walks for bathroom breaks are fine, but nothing more strenuous. Small amounts of drainage from the incision site are normal for the first few days, and mild swelling should resolve quickly. Watch for signs like pale gums, vomiting, loss of appetite, difficulty urinating, or bleeding from the incision, which would warrant an immediate call to your vet.
Timing Matters
Most vets recommend surgery before two years of age. The cancer risk associated with a retained testicle increases as the dog gets older, so there’s no advantage to waiting. If your puppy is younger than six months and only one testicle is visible, your vet may suggest monitoring for a short period since late descent occasionally happens. But after six months, the chances of the testicle dropping on its own are essentially zero.
If you’re noticing this in an older dog who was never treated, the surgery is still worth doing. The retained testicle has been sitting at elevated body temperature for years at that point, and the cumulative cancer risk is substantial. Your vet may recommend an ultrasound first to check for any masses that have already developed, which would affect the surgical approach.