Erosion and infection are major complications following implantation of the AMS 800 artificial urinary sphincter. In the present study, 17 patients with this prosthesis were investigated urodynamically. All were continent. Urethral pressure profilometry showed a significantly lower intra-urethral pressure than would have been expected from the pressure installed in the pressure regulating balloon during operation. The maximal urethral pressure was 69.9% of the expected pressure and did not vary significantly in relation to the various balloon pressures. The mean maximal urethral pressure (+/- SEM) between the cuff when inflated and when deflated was also significantly different, although the absolute difference was small (14.1 cm H2O +/- 10.0). There was good correlation between maximal urethral pressure and urethral leakage pressure as measured by retrograde perfusion sphincterometry. The mean functional urethral length became significantly longer after implantation of the cuff. A combination of low urethral pressure and increased functional length may help to prevent cuff erosion and maintain continence.