A double blind comparative clinical trial was performed with allylestrenol (AE) and chlormadinone acetate (CMA) to investigate the clinical efficacy of AE on prostatic hypertrophy. Both drugs were administered orally for 12-16 weeks in a daily dose of 50 mg. With both drugs marked improvement of disorders of micturition and a slight decrease in the size of the hypertrophied prostatic node were observed. No significant difference was observed between the two drugs in the overall efficacy of the treatments. Significant improvement of practically all parameters used for evaluation of results was observed with both drugs following treatment. Ultrasonotomographic examination revealed diminution of the size of the prostatic node and x-ray examination of the ureter showed improvement in elevation of the fundus of the bladder. These improvements were better after CMA treatment than after AE treatment. With all other parameters used no significant difference was observed between the two drugs. Mild adverse effects such as loss of sexual desire and potency were observed in a few cases. The incidence of side-effects was lower following AE treatment, and the incidence of loss of sexual desire and potency was significantly lower after AE than after CMA. Taking into consideration efficacy and safety of the treatments, no significant difference was observed in usefulness between the two drugs, and we were able to confirm the usefulness of AE for the conservative treatment of prostatic hypertrophy.