Preliminary results of a long-term, prospective therapeutic trial with mebendazole in 28 patients mainly with inoperable echinococcosis are reported. The course of disease was monitored closely and plasma mebendazole levels were checked regularly. A major problem is the lack of reliable methods for defining and measuring early success or failure of therapy. Most of the patients improved clinically. No marked change of parasitic lesions was observed except in one patient with constantly low plasma mebendazole levels who deteriorated. No serious side effects occurred. The preliminary results are encouraging and suggest that mebendazole has a "parasitostatic" effect on larval growth.