Six patients with polycystic hydatid disease (PHD) were treated with 10 mg kg-1 day-1 albendazole. One patient was treated continuously for eight months and another for three months. In three other patients treatment was discontinuous, consisting of a series of at least three 30-day cycles separated by 15 days without treatment. The last patient was treated continuously with 12 mg kg-1 day-1 albendazole for 51 days and then with three 30-day cycles of treatment with 10 mg kg-1 day-1 separated by 15-day drug-free intervals. Follow-up ranged from 10-30 months. Considerable clinical improvement and cyst reduction or disappearance occurred in four patients. Clinical improvement, but no changes in the hepatic alterations detected by computerized tomography, occurred in the other two patients, although a pulmonary cyst disappeared in one of them. Adverse effects were proteinuria, alopecia, leucopenia, itching and discrete elevation in aspartate transaminase, all of them reversed after the end of treatment. These results indicate that albendazole is effective for the treatment of PHD.