This study was designed to evaluate the safety and effectiveness of testosterone therapy for clinical symptoms of hypogonadism (low libido, low mood, low energy, loss of appetite/weight) in human immunodeficiency virus-positive men with CD4 cell counts less than 400 cells/mm3 and deficient or low normal serum testosterone levels. The trial consisted of 8 weeks of open treatment with 400 mg of intramuscular testosterone cypionate biweekly. Responders were maintained at this dosage for another 4 weeks and then were randomized in a double-blind, placebo-controlled, 6-week discontinuation trial. Of the 112 men who completed at least 8 weeks of treatment, 102 (91%) were rated as responders on a global assessment of sexual desire/function. Of the 34 study completers with major depressive disorder and/or dysthymia, 79% reported significant improvement in mood at week 8. Average weight change was a gain of 3.7 pounds, with 45% gaining more than 5 pounds. Eighty-four men entered and 77 completed the double-blind phase; of these, 78% of completers randomized to testosterone and 13% randomized to placebo maintained their response. No significant medical or immunologic adverse effects were identified. Testosterone therapy was well tolerated and effective in ameliorating symptoms of clinical hypogonadism, and equally so for men with and without testosterone deficiency. For patients with major depression and/or dysthymia, improvement was equal to that achieved with standard antidepressants.