A patient with an Echinococcus granulosus cyst of the liver was treated with mebendazole for 94 days before operation. The serum levels of mebendazole varied from 39.4–274 ng/ml. After operation, cyst materials were inoculated into mice which developed hydatid cysts 10 months later. Intestinal absorption of mebendazole is poor and variable, and determination of serum concentrations is necessary during treatment. No apparently successful cases of drug treatment of E. granulosus infection have been verified by animal inoculation of cyst material; therefore, surgery must still be considered the treatment of choice. It is recommended that mebendazole be given prophylactically to prevent spread of the disease at operation.