In a 12-month period 100 patients with clinical evidence of acute disease of the gallbladder were studied in hospital by grey-scale ultrasonography. During the same hospital admission it was possible to correlate results of ultrasonography with operative findings in 66 patients. In 52 patients the ultrasonographic diagnosis of gallstones was proved to be correct. There were no false-positive results. In seven patients the ultrasonographic report of a normal gallbladder without stones was also confirmed. In two patients, the report of a normal gallbladder without stones was erroneous. In two more patients the scan was indeterminate and stones were found at operation. In the remaining three patients echogenic material was reported and at operation minute stones and "sludge" were found. No complications resulted from the ultrasonography. The study showed that grey-scale ultrasonography is a reliable, rapid and safe technique for detecting gallstones in patients with a clinical diagnosis of acute cholecystitis.