Their research, published in Saturday's issue of The Lancet, makes painful reading for any male cyclist keen on off-road biking.
The doctors, from the departments of radiology and urology at University Hospital Innsbruck, carried out ultrasonic scans on 45 amateur mountain bikers aged 16 to 44.
All of them cycled for at least two hours a day, six days a week, covering a total distance of at least 5,000 kilometers (2,500 miles) a year.
Virtually all of the bikers 96 percent had scrotal abnormalities, especially benign, stone-like calcifications called scrotoliths. Half of them said their scrotum felt tender and uncomfortable.
Compared with a control group of 31 healthy medical students who rode bikes but did not go mountain biking, the off-road group suffered three times as many spermatoceles, an abnormality in which sperm-containing cysts form in the scrotum.
This suggests that biking causes microscopic damage to the epididymis, the tightly coiled tubes folded against the back of each testis where sperm are stored and mature, says researcher leader Ferdinand Frauscher.
"We assume that the abnormalities of scrotal contents detected in our study are essentially a saddle-related problem," he says.
Riders can help prevent such damage by wearing padded shorts or a padded saddle, taking frequent rests during each ride and ensuring that the saddle is at the correct height and angled horizontally or upward in front.
"Furthermore, new shock-absorbent saddles and full-suspension bikes might help reduce saddle vibration and microtraumatisation so that mountain biking remains a relatively safe and healthy sport," Frauscher says.
The study notes previous research that says excessive biking and a hard saddle can cause injury to nerves in the pudenda.
"If blood supply to the penis is compromised, male erectile dysfunction may develop," it warns.