The assessment of a ten-year clinical trial of continuous therapy in eight patients revealed further evidence of a significant therapeutic effect of mebendazole on alveolar hydatid disease. Life-expectancy was increased when compared to untreated historical controls, especially in the patients over 55 years of age. All symptomatic patients showed subjective improvement. In four patients, three had a 50% or greater reduction in the diameter of massive hepatic lesions, and in the fourth, progressively enlarging metastases were arrested. Fall in the IHA titre suggested that the causative organism had been destroyed in two additional patients. Of greater significance was the absence of progression of the disease process as measured by changes in the size of the hepatic lesion or lack of development of distant metastases in patients under therapy. In contrast, progressive enlargement of hepatic lesions or the appearance of distant metastases were cardinal features of untreated cases (15 of the 16 cases followed). In vivo determination of viability of tissues of the larval Echinococcus multilocularis from patients receiving long-term therapy was considered important in evaluating efficacy of the drug. Such tissues, obtained by autopsy from two patients under continuous therapy for four and ten years, failed to proliferate when inoculated into rodents (red-backed voles), whereas similar inoculations from untreated patients or those receiving 15 months' or less of therapy brought about production of vesicles in rodents in eight of 11 tests (73%). These two deaths, unrelated to therapy, resulted from late fibrotic constriction of end-stage parasitic lesions about the portal vein and major bile ducts. The clinical findings in combination with negative in vivo tests and other data indicate that the mebendazole therapy significantly alters the clinical course of alveolar hydatid disease. The evidence strongly indicates that long-term therapy may eventually have a lethal effect on the larval cestode in advanced disease.