The effect of high dosage therapy with Mebendazole was studied in 5 patients with cystic echinococcosis and 2 patients with alveolar echinococcosis involving several organs. Follow-up studies at regular intervals from 7 to 20 months indicated an improvement of the pathological findings initially observed. This could clearly be demonstrated by the radiographic findings in cases with pulmonary involvement. The desintegration or dissolution of pulmonary cysts was observed within a few months after treatment with Mebendazole had begun. However, consolidation of previously emptied cysts occurred during the follow-up later. Allergic reactions were observed after commencement of drug therapy in two cases. These were probably due to spillage of antigenic material. The following treatment periods were tolerated well. No serious toxic side effects were observed in the other cases described here.