The effects of the antiandrogen drugs cyproterone acetate (CPA) and spironolactone on hair growth and androgen levels were compared in a randomized study of 48 hirsute women. Twenty six subjects completed 6 months of therapy with 100 mg/day CPA and 19 subjects completed 6 months of 100 mg/day spironolactone. All except 10 subjects received concomitant estrogen therapy. Measured objectively, total hair diameter fell by 17.1% with spironolactone (P < 0.001), and by 16.8% with CPA (P < 0.001). The diameter of the hair medulla fell by 17.8% with spironolactone (P < 0.01), and by 31.7% with CPA (P < 0.001). There was no difference between the drugs in their effect on hair diameter. Plasma testosterone levels also fell significantly with both drugs. As a subjective assessment of treatment efficacy, the frequency with which subjects performed cosmetic measures was recorded. This fell by 38% with spironolactone and by 44.7% with CPA (P < 0.001 both drugs), and again there was no difference between the drugs. Side effects caused cessation of treatment in one subject taking CPA and two subjects taking spironolactone, and milder side effects were noted in two further subjects from each treatment group. We conclude that spironolactone and CPA, in the dosages used in this study, are effective and well tolerated agents for the treatment of hirsutism, and that neither drug demonstrates a particular advantage over the other.