Eleven heterogeneous patients with inoperable hydatid disease were treated with high doses of oral mebendazole, up to 200 mg/kg/day for 16 to 48 weeks. Toxicity was not encountered. Despite careful assessment it proved difficult critically to evaluate the efficacy of mebendazole. In four patients chemotherapy appeared to have been at least partially successful and success seems to have correlated with serum levels in excess of 100 ng/ml one to three hours after dosage. The problems of assessment are discussed and the need for controlled trials of mebendazole in hydatid disease are stressed.