An objective evaluation of the anti-androgen effects of spironolactone was performed in a consecutive series of 12 hirsute patients receiving a daily dose of 150 mg; nine completed the study. Using a computer assisted image analyser, hair diameter on two weekly shavings decreased significantly over a 12 month period in three of the patients, although growth rate and mean diameter did not change in the group as a whole. Plasma testosterone fell significantly to a mean of 53% of basal levels. The mean free testosterone (derived) fell significantly to 64% of basal by the sixth month (P = less than 0.005) and remained significantly depressed the remainder of the study. There was subjective benefit in hair growth and greasiness and a significant reduction in the semi-objective Ferriman-Gallwey index in nine of 10 subjects assessed for at least 9 months. We conclude that although spironolactone was not consistently successful, it may represent effective therapy for a sub group of patients with hirsutism.