Since an erection requires a precise sequence of events, impotence can occur when any of the events is disrupted. The sequence includes nerve impulses in the brain, spinal column, and area around the penis, and response in muscles, fibrous tissues, veins, and arteries in and near the corpora cavernosa.

Damage to nerves, arteries, smooth muscles, and fibrous tissues, often as a result of disease, is the most common cause of impotence. Diseases--such as diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease--account for about 70 percent of impotence cases. Between 35 and 50 percent of men with diabetes experience impotence.

Also, surgery (especially radical prostate and bladder surgery for cancer) can injure nerves and arteries near the penis, causing impotence. Injury to the penis, spinal cord, prostate, bladder, and pelvis can lead to impotence by harming nerves, smooth muscles, arteries, and fibrous tissues of the corpora cavernosa.

In addition, many common medicines--blood pressure drugs, antihistamines, antidepressants, tranquilizers, appetite suppressants, and cimetidine (an ulcer drug)--can produce impotence as a side effect.

Experts believe that psychological factors such as stress, anxiety, guilt, depression, low self-esteem, and fear of sexual failure cause 10 to 20 percent of impotence cases. Men with a physical cause for impotence frequently experience the same sort of psychological reactions (stress, anxiety, guilt, depression).