There have been anecdotal reports of a decrease in penile size in men with erectile dysfunction (ED) after nerve-sparing radical retropubic prostatectomy (NSRRP). Penile circumference and length measurements are obtained by one physician from 100 men, age 47 to 74, who presented at various intervals (1.7&endash;27.6 months) for the treatment of ED after NSRRP from 1994 through 1997. All patients were asked to complete a brief male sexual function inventory at their initial visit. Penile measurements were obtained both in the flaccid and erect states, with erections being induced with intracorporal injections of Trimix. The sexual inventory scores were compared with those of an age-matched control cohort of 130 men presenting for evaluation of ED during the same time period and 132 age-matched men who completed the inventory at the time of a prostate screening. By self-report, men experiencing ED after NSRRP had better libido but more severe ED than men presenting with ED of other causes. There was a decrease in all penile dimensions after NSRRP. The flaccid and erect measurements of length and circumference decreased 8% and 9%, respectively after surgery (p > 0.05). The most substantial change occurred between the first 4 and 8 months postoperatively. The average change in volume between the first 4 and 8 months was 19% to 22% in the flaccid and erect state, respectively. There is a significant decrease in penile size in men with ED after NSRRP. The etiology may be denervation smooth muscle atrophy, apoptosis, or hypoxia-induced damage to the corpora. Further research is needed to elucidate the nature of these postoperative changes.