Mebendazole was given as definitive therapy to four patients with cystic hydatid disease of the liver who were later explored surgically. Viable hydatid material was removed from two patients after continuous therapy with 40 mg/kg/day for eight and three months respectively. The hydatid material was necrotic in the of her two patients. This drug appeared to be ineffective as definitive treatment of two of these four patients with established hydatid disease of the liver.