Fifty patients with 81 abdominal hydatid cysts were followed with ultrasound during and after treatment with albendazole. In 61 cysts (75%), regressive changes were observed after treatment. Detachment of the membrane and change to a solid pattern in anechoic cysts were observed. Disappearance of septa or change to a solid pattern in anechoic cysts with intracystic septation were also found. There was an increase of hyperechoic structures in cysts with a mixed pattern. Follow-up ultrasound examination showed disappearance of 7 cysts, while anechoic structures reappeared in five cases. In anechoic cysts, regressive changes due to albendazole seem to be permanent, but in cysts with a mixed pattern recurrences are sometimes observed.