In a double-blind study of four patients with hereditary angioedema, the efficacy of methyltestosterone (taken daily in 10-mg linguet form) in preventing attacks was shown. There were 19 episodes during 11.8 months of placebo administration, compared with only four attacks during the 46 months of cumulative methyltestosterone treatment (P less than 0.001). The mean serum C4 protein level was twice as high in all patients when they were taking the drug (176 +/- 36 mug/ml) as compared with the placebo (84 +/- 21 mug/ml), and rose to normal range in three of four patients.