Levonorgestrel, as used in oral contraceptives, has antiovulatory activity at doses far lower than those producing androgenic effects. Triphasil, containing levonorgestrel was compared with Diane, containing cyproterone acetate in a trial of acne treatment. Twenty closely matched patients were alternatively allocated to 6 months of Triphasil or Diane treatment. Both groups had a 72% reduction in acne counts. Assays of total testosterone, androgen index, free testosterone, dehydroepiandrosterone sulphate and androstenedione showed significant reduction on treatment and sex hormone binding globulin was raised. All hormonal changes were more marked in the Diane group. Side effects caused only one withdrawal from the trial.