The clinical reports on Sildenafil sulfate (Viagra) are mainly based on individual observations. However, there is a paucity of objective studies in the literature. In order to objectively examine the effect of Sildenafil, a SS (Sexual Stimulation)-Penogram that is a non-invasive, simple and physiologic method was developed using a radioisotope (RI). One hundred and four SS-penograms were performed on patients who had a documented erectile dysfunction (ED) lasting for more than 6 months. After an intravenous injection of 99mTc-RBC (15 mCi), the first penogram was taken immediately after sexual stimulation, which was done by 30 minutes of erotic videotape viewing. Forty minutes after administering 25 to 100 mg of Sildenafil, a second penogram was taken. The characteristics of each penogram were analyzed according to a previously reported method. The results were graded as follows; Type I(normal function; 5 min or more of peak erectile response with an induction period of 1 to 6 min), Type II-A (impossible function type; i.e., showing less than 2 times the basal radioactivity level), Type II-B (the unstable type; showing less than 5 min of peak erectile response), and Type II-C (the delayed type; which showed a delay of more than 15 min after the start of sexual stimulation). The patients were grouped according to their response after Sildenafil administration, and the effect of Sildenafil was assessed by comparing the radioactivity from between 7 to 22 minutes and the changes in the characteristics of the penogram. The mean age of the patients was 44.9 ± 10.2 (23 - 68) years. In the first penogram, Type I was found in 12 patients, and Type II-A in 14, Type II-B in73, Type II-C in 1 and a mixed (II-B + C) type was found in 4 patients. A second penogram after Sildenafil administration, showed Type I in 46 patients, and Type II-A in 10, Type II-B in 46 and a mixed type was found in 2 patients. The responses after Sildenafil were categorized as follows: 1) An excellent response group (consisting of 56 patients-53.9%); Those who showed greater than 50% increase in the RI area after Sildenafil treatment. 2) A good response group consisting of (23 patients-22.1%); i.e., those who showed a less than 50% but greater than a 20% increase in the RI area after Sildenafil administration. 3) A borderline group (consisting of 15 patients-14.4%); showing less than a 20% change in the RI area after Sildenafil treatment. 4) non-response group (consisting of 10 patients-9.6%).