Twenty-three patients with cystic or alveolar echinococcosis were treated with mebendazole in a prospective manner (50 mg per kg bodyweight per day per os). In cases of cystic echinococcosis, mebendazole was given for one month. Repeated consolidation courses were performed at regular intervals during the follow-up. In cases of alveolar echinococcosis, the intermittent scheme was also applied, but later on the drug was administered continuously. The follow-up period ranged from 4–40 months after the initiation of mebendazole treatment. Regular follow-up examinations during and following treatment were performed, and the effect of the chemotherapy was assessed by sensitive methods, i. e. conventional radiology, ultrasonics, computerized tomography. A reduction in cyst size after treatment with mebendazole was clearly visible only in patients with pulmonary lesions. Of the six patients with lung cysts, five were found to have improved considerably, as judged by conventional chest X-ray. Only two of the 11 patients with liver involvement showed a considerable reduction in cyst size. Regression in patients with alyeolar echinococcosis was moderate in two of seven cases. However, improvement of indirect parameters (e. g. liver enlargement, tenderness, cholestasis, hepatocellular damage or weight loss) was considerable in those patients presenting these clinical symptoms. Allergic and cardiopulmonary side-effects were only observed in patients presenting lung cysts, and were most probably due to the rupture of cysts after the administration of mebendazole; they always occurred during the first treatment cycle. Haematotoxic effects were observed in one patient after the fifth treatment cycle with mebendazole.