We previously examined the effects of overdistension on the neuromuscular system of canine urinary bladders and reported that bladder overdistension led to nerve degeneration and subsequent supersensitivity through a decrease of blood supply to the bladder. We have accordingly in this study evaluated these changes in human subjects with infravesical obstruction. The responses to acetylcholine of bladder strips obtained from patients with detrusor instability were not significantly different from those of bladder strips from patients without detrusor instability, but the dose-response curve of these groups showed a shift to the right compared to that of the unobstructed control patients. As compared with the response of bladder strips in patients without an episode of retention, the response in patients who received prostatectomy within 30 days demonstrated no significant difference, although in patients who received prostatectomy after more than 30 days there was a statistical difference. These results indicated a significant decrease in sensitivity of the detrusor muscle in patients with infravesical obstruction and suggest that bladder overdistension caused by infravesical obstruction may lead to supersensitivity of the detrusor muscle secondary to denervation.