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Evidence that the larval stage of Echinococcus multilocularis in humans is killed by chemotherapy is presented in a review of our 17-year experience with treatment of alveolar hydatid disease in Alaska. The efficacy of chemotherapy was assessed with use of an in vivo assay of parasite viability by means of inoculation of voles, immunohistochemical tests, and histopathologic findings. Of 14 tests performed for nine patients, 12 in vivo assays (86%) were negative after chemotherapy, while only two (17%) of 12 vole tests for seven untreated patients were negative. Regression or arrest of growth of metastatic and primary hepatic lesions, together with their partial-to-complete calcification and prolonged survival times has been observed among patients treated with the benzimidazole compounds. For six who received appropriate chemotherapy, treatment has been discontinued for an average of 4.6 years (range, 3-7 years) without an increase in lesion size or other evidence of reactivation.