Fifty-three patients with single, multiple and/or multi-organ hydatid cysts were treated with mebendazole, in varying dosages (30-70 mg/kg.day) and over varying periods (6-24 months). Treatment failure was recorded if the mebendazole had no apparent effect on cyst morphology (monitored by radiology, ultrasonography and computed tomography) or only a parasitostatic effect (characterized by mixed and transitory, hypo- and hyper-echoic changes in the lesions). Cure was indicated by a parasitocidal effect (characterized by a totally echogenic picture, increased density of the cyst contents, reduction in the size or complete disappearance of the cysts, complete detachment of the endocysts and/or calcification of the cyst wall). Treatment failure was seen in about 40% of the patients and cure in about 38%; the other 23% had an intermediate result considered as an improvement. Cure rates increased both with dosage and duration, daily dosages of 60-70, 50 and 30-40 mg/kg curing 48%, 33% and 25%, respectively. The pre-treatment condition of any cysts should be taken into consideration when determining dosage and treatment duration.