Clinical effects of intramuscular injection of 250mg testosterone cypionate every 2-3 weeks were investigated in 15 impotent patients with low or low normal serum testosterone level (Hypo-gonadotrophic hypogonadism was excluded). The results were obtained as follows. 1. Among 15 patients, 7(46.7%) showed markedly improved potency, 6(40%) partially improved potency and 2(13.7%) no improvement of potency. There was no correlation between effec-tiveness of testosterone replacement and age, testicular size, serum testosterone level or LH, FSH level. 2. Among 12 patients who had showed improved potency, 8(66.7%) complained of redeveloped decrease in potency during testosterone replacement. In conclusion, testosterone cypionate for treatment of the impotent patients with low or low normal serum testosterone level was effective, but further studies are necessary to investigate cause of redeveloped decrease in potency during testosterone replacement.