The effect of oral cyproterone acetate on hair density and diameter was studied in a clinically homogeneous group of 11 women with idiopathic hirsutism. Therapy was continued for 12 cycles of treatment. Hair density and mean hair diameter on the chin and left upper leg were measured before, immediately after and 3 months after therapy. Generally, the effect of treatment consisted of a quantitative reduction of both parameters, but the effect was not permanent after termination of therapy. Concurrently, the possible influence on plasma androgen was studied. No significant differences were found between the concentrations of plasma testosterone, dehydroepiandrosterone and androstenedione before and after treatment.