This study was undertaken to identify the cause of impotence in men undergoing radical prostatectomy, with the hope that this information may provide insight into the possible prevention of this complication. The autonomic innervation of the corpora cavernosa in the male fetus and newborn was traced to determine the topographical relationship between the pelvic nerve plexus, and the prostate, urethra and urogenital diaphragm. We have demonstrated that the branches of the pelvic plexus that innervate the corpora cavernosa are situated between the rectum and urethra, and penetrate the urogenital diaphragm near or in the muscular wall of the urethra. Injuries to the pelvic plexus can occur in 2 ways: 1) during division of the lateral pedicle and 2) at the time of apical dissection with transection of the urethra. Thirty-one men who underwent radical retropubic prostatectomy were evaluated to determine risk factors that correlated with postoperative impotence: 5 (16 per cent) were fully potent, 7 (23 per cent) had partial erections that were inadequate for sexual intercourse and 19 (61 per cent) had total erectile impotence. The 2 factors that had a favorable influence on postoperative potency were age and pathologic stage of the lesion: 31 per cent of the patients less than 60 years old were potent versus only 6 per cent of the patients more than 60 years, while 33 per cent of the patients with tumor microscopically confined to the prostatic capsule were potent versus only 5 per cent of those with capsular penetration. When the factors of age and capsular penetration were combined 60 per cent of the men less than 60 years old who had an intact prostatic capsule were potent. Arterial insufficiency and psychogenic factors were excluded as major contributing factors by the finding of normal penile blood flow and absence of nocturnal penile tumescence in the impotent patients. We conclude that impotence after radical prostatectomy results from injury to the pelvic nerve plexus that provides autonomic innervation to the corpora cavernosa. Further studies will be necessary to determine whether refinements in surgical technique, especially during ligation of the lateral pedicle and apical dissection, can prevent this complication.