The research suggests frequent jolts and vibration caused by biking over rough terrain may cause abnormalities, including small scars within the scrotum and impaired sperm production.
The abnormalities were found in professional mountain bikers and other "extreme" bikers who logged at least 3,000 miles yearly - or an average of more than two hours a day, six days a week.
Dr. Ferdinand Frauscher, a urology-radiology specialist at University Hospital in Innsbruck, Austria, said he studied about 55 avid mountain bikers and found nearly 90 percent had low sperm counts and scrotal abnormalities.
Only 26 percent of the 35 non-bikers he studied had similar damage, according to research presented at this week's annual meeting of the Radiological Society of North America.
Whether the abnormalities were severe enough to make fathering a child difficult is uncertain, though some of the bikers studied had already experienced difficulty conceiving, Frauscher said.
Participants were aged 17 to 44.
His study looked at fertility rather than impotence, which was linked to recreational cycling in research heavily publicized in 1997. The earlier findings, by Boston University impotence specialist Dr. Irwin Goldstein, were construed by many cycling aficionados to suggest that men should avoid any cycling sports.
Some doctors thought Goldstein's findings were overstated, but the issue has prompted a mini-industry of bicycle seats designed to avoid the compression of penile arteries that Goldstein said occurs during cycling.
Such problems may occur on narrow, racing-type seats, Frauscher said. Some newer, wider designs feature holes or gaps to avoid pressure, but these likely would have no effect on the scrotal damage found in the Austrian study, which may be caused by jolting over rough terrain rather than artery compression, Frauscher said.
Frauscher said men shouldn't avoid mountain biking because of the study, but should perhaps consider investing in bikes with shock absorbers or suspension systems designed to reduce the jolting.
Stanford University urologist Dr. Robert Kessler said he was skeptical of Frauscher's findings. Scrotal varicose veins, which were among the abnormalities Frauscher linked to mountain biking, are usually congenital and not linked to trauma, Kessler said.
"It doesn't make sense," Kessler said.
Dr. Eduardo Randrup, a urologist at Ochsner Clinic in New Orleans, said the link is plausible but not necessarily cause for alarm. The damage Frauscher found "may well be reversible" and likely would not occur from recreational cycling, Randrup said.
Dr. Sangili Chandran, a sports medicine specialist at Christ Hospital and Medical Center in the Chicago suburb of Oak Lawn, said other studies have found similar results but, like Frauscher's, have been too small to be conclusive.
"The theory is there, the proof I'm still looking for," Chandran said.
Even if the results are corroborated in future studies, very few mountain biking enthusiasts are logging enough miles to worry about any fertility impairment, Chandran said.
By Lindsey Tanner