We have prospectively studied the evolution of 13 patients with liver hydatid cysts treated with mebendazole. Two patients also had peritoneal hydatid disease in addition to liver cysts. One of the two also had a retrovesical cyst. Liver cysts disappeared in three patients. In one, the liver cyst reappeared and in another a new cyst was observed after treatment was stopped. Therapy was most effective in young patients, in small cysts, in one case of peritoneal hydatid disease and in one case of a retrovesical cyst. Side effects warranted interruption of therapy in three cases, although we cannot be sure that they were due to mebendazole treatment. We recommend long-term patient follow-up once the cyst can no longer be identified by ultrasound.