Fifty-four patients with benign prostatic hypertrophy underwent transrectal ultrasonography before and after treatment with 50 mg/day of chlormadinone acetate (CMA). Weight of both inner and outer prostate glands had decreased significantly by the end of the treatment. Judging from changes in weight of the inner prostate gland, CMA is most effective in cases of mild-to-moderate prostatic hypertrophy in which the inner gland weighed less than 30 grams. Six of our patients received CMA for a prolonged period of time: average, 22 months. Weight of the inner prostate gland appeared to decrease significantly during the first 10 months in all six cases. After 10 months, however, prostate weight in two patients remained unchanged and increased again in the other four. Treatment was discontinued in eight cases. Weight of the inner prostate gland decreased significantly after an average of about eight months of treatment, but then increased to its pre-treatment level after an average of about eight more months. These results suggest that the prostate may often return to its pre-treatment weight in spite of continuous CMA treatment, and, further, that it may be important to monitor the effect of CAM treatment to decide whether or not continuation of treatment is warranted. CAM treatment should be resumed while its effect remains, after interruption of treatment.