In his office at Boston University Medical Center, Dr. Goldstein treats six men a week on average, whom, he says, have become impotent from riding a bicycle. Overall, this urologist, estimates there are about 100,000 men who have lost their ability to get or maintain satisfactory erections because of penile damage inflicted by either the bike's top tube or its saddle.

This is the message Dr. Goldstein is vociferously delivering in media interviews, university lectures and medical seminars nationwide. Considered one of the country's leaders in impotency research, Dr. Goldstein helped engineer the shift in treating male sexual dysfuntion from a psychological problem to a chiefly physical one.

Few have paid attention to Dr. Goldstein's warnings. In fact, within cycling circles he was shrugged off and regarded as somewhat of a kook. But as he developed a nationwide reputation, he began to see more impotent riders who couldn't point to any one accident as a potential cause. This eventually led him to explore the role saddle compression may play in cycling.

Richard Belmont, a 54 year-old attorney, never had any sexual-performance problems. But after cycling for almost 200 miles during a two-day charity ride last summer, he suddenly became impotent. Pedaling a new road bike equipped with a racing saddle and an aero bar, he noticed that his penis was numb for most of the event. After experiencing erection difficulties during the next six months, he finally visited Dr. Goldstein, who spotted arterial damage at the base of the shaft.

Greg Jackson , a 33 year-old computer programmer, always kept in great shape. But after a long mountain bike on rough terrain, he experienced pain that went beyond the usual aches. Its epicenter was near his penis, which wouldn't get as hard or as erect as it used to. When he eventually consulted Dr. Goldstein, he learned that his main penile artery had become clogged.

To understand what happened to these men ( and, most important, how you can keep it from happening to you) you need to know a few things about the male anatomy. The penis is a hydraulic system. During sexual stimulation, its twin chambers fill with blood until it is firm and erect. All of this essential wiring (nerves, blood vessels) is funneled through an area called the perineum, which lies between the sit bones. Dr Goldstein explains that your weight is being focused between these sit bones, and that's where the arteries and nerves that feed the penis are located.

To study how such pressure might compromise sexual performance, Dr. Goldstein set to measure the degree of compression in the cavernosal artery that feeds the penis. He used three different methods (x-ray, ultrasound and arterial) to quage the effects of sitting on a narrow unpadded saddle, a wide padded saddle and a chair. From a test group of 100 impotent patients studied over 10 months, he determined that it takes only 11% of person's body weight to compress the artery. He measured a 66% average reduction in blood flow through the artery when subjects were on the skinny saddle, 25% when on the wide saddle and no change when seated on the chair. This led Dr. Goldstein to conclude that, for a given weight, the wider the surface area of the seat, the less the chance of compression.

If you add the insult of chronic compression to an older man who has the beginnings of arteriosclerosis, hypertension, a cholestrerol and weight problem, then the straw won't necessarily rebound back as it would in a younger, healthy man, "Dr. Goldstein adds. I think that's where this fits in."

An associate professor of urology at the University of Southern California and the director of The Male Clinic in Santa Monica, California, Dr. Harin Padma-Nathan says he treats 50 to 100 cases of what he terms bicycle related impotency annually.

Typically, the patient is either a roadie who has logged many, many miles on an unpadded saddle that was set fairly high, or a mountain biker who has been riding off road trails aggressively. Both types are also usually quite lean, meaning they have even less cushioning in their nether regions.

As Richard Belmont, the patient who experienced penile numbness on a 200 mile ride puts it: "The thing that angers me most in all of this is my own stupidity. I had absolutely no idea I could become impotent by riding a bicycle. If I had known, I would have ridden smarter. I think a lot of guys out there need to get that message."