There have not been studies assessing the effects of chronic testosterone cypionate (TC) therapy on circulating levels of testosterone (T), estradiol (E2), free T, bioavailable T (BAT), luteinizing hormone (LH), and sexual function in impotent men with low T levels. This study was a double-blind crossover using 200 mg of TC or placebo given intramuscularly every 14 days for six injections and the other medication given for six doses. Blood was drawn before each injection. Mean concentrations of T, E2, free T, and BAT were the same on TC or on placebo, but serum LH was significantly suppressed during intramuscular TC. With TC statistically significant improvements in libido and in potency were noted. Five of the men were able to have vaginal sex while taking TC. TC injections every 14 days do not appear to maintain increased T concentrations for 2 full weeks, and other dosage/injection schedules are being evaluated, but there were improvements in libido and potency.