The mechanical reliability of AMS hydraulic penile prostheses implanted in 203 patients from April 1985 to June 1995 were evaluated. AMS Hydroflex prosthesis showed the highest incidence of mechanical failure (18.8%; 6/32 patients) during a mean follow-up period of 94.5 (64-117) months. Mean functioning time of the prostheses until malfunction was 50 (0-100) months. Bilateral fractures at junction of rear reservior and inflation chamber were found in 3 patients. AMS Dynaflex had a failure rate of 2.4% (2/85 patients) for an average of 35.3 (1-59) months. One patient showed complete fracture of silicone ball covering the proximal end of rear reservior onto which rear tip extenders are snapped. Regarding 3-piece inflatable prosthesis, AMS 700, AMS 700CX and AMS Ultrex had failure rates of 11.1% (1/9 patients), 10.5% (2/19 patients) and 4.0% (1/25 patients) during a mean follow-up period of 116.4 (103-125), 79.0 (60-94) and 44.4 (22-57) months, respectively. The verified causes of the mechanical failures were a tiny rupture of the cylinder in one case of AMS 700, an incomplete fracture of input tube in one case of AMS 700CX and a pump malfunction in one case of Ultrex. However, none of 33 cases of AMS 700CXM showed mechanical failure for an average of 21.3 (2-43) months. Therefore, AMS 700CXM and Ultrex seem to be very reliable, and the reliability of AMS Dynaflex was much higher than that of AMS Hydroflex. However, the long-term reliability of these devices needs more time to be determined.