The sonographic appearance of 141 hepatic and abdominal hydatid cysts was studied in 63 patients undergoing therapy with benzoimidazolic compounds. The cysts were evaluated for size, margins of the cyst wall, and echo pattern of cyst contents to establish the response to the drugs. Changes seen on ultrasound (US) scans were classified as decrease in size, detachment of the cyst membrane, and appearance of echogenic material (matrix) in the cyst cavity. These findings, particularly the diffusely hyperechoic pattern, are explained on the basis of the action of the drugs. Twenty-one patients underwent surgery at the end of treatment. The US patterns of treated hydatid cysts are similar to those seen, though more rarely, in cases of spontaneous parasite involution in long-standing hydatidosis.