Since 1967, 40 patients with hydatid disease of the liver have been treated at our hospital. Diagnosis was made using clinical criteria, serology, skin tests, and imaging techniques. Thirty-five patients were operated upon. In 18 patients the cyst was uncomplicated (Group I), and in 17 the cyst was infected or communicated with the biliary tract (Group II). Three forms of surgical treatment were used: A) cyst evacuation, scolicidal irrigation, and primary cyst closure, B) evacuation, irrigation, and external drainage, and C) complete or partial cyst resection. Mebendazole was used in six patients, four of whom were also treated surgically. In Group I, one of 11 patients (8%) treated by primary closure had complications, versus four of five patients (80%) treated with external drainage (p less than 0.001). Mean postoperative hospital stay for these two groups was 11.8 versus 20.8 days, respectively (p less than 0.001). Complication rates in Group II were higher, and were evenly distributed among treatments. Patients have been followed yearly, with a median follow-up of 5 years. Active hydatid disease has been found in three patients, who all had known residual disease at initial operation. The best treatment for an uncomplicated hydatid liver cyst is evacuation, scolicidal irrigation, and primary closure. External drainage is used for infected cysts or those communicating with the biliary tract, and excision for extrahepatic and peripheral, easily resectable cysts. Mebendazole is used for intraperitoneal spillage of cyst contents and in patients with inoperable disease.