Ultrasound examinations of 563 patients with right upper quadrant pain and a clinical suspicion of acute cholecystitis were reviewed. In 31 patients, a tender, dilated gall-bladder with a thick (more than 4 mm) partly hypoechoic wall without any detectable calculi was found on the emergency examination. This was interpreted as due to acute acalculous cholecystitis. None of the patients was critically ill. Twenty-one of the patients had follow-up studies with either oral cholecystography, cholangiography, or ultrasound. Fourteen of the 21 had gall-bladder calculi while seven did not. These seven patients presumably represent the true frequency (1.2%) of acute acalculous cholecystitis in this clinical setting. In five other patients with an initial diagnosis of acute acalculous cholecystitis the gall-bladder wall thickening probably was secondary to concomitant pancreatitis, appendicitis, hepatitis or peptic ulcer disease. A meticulous and careful search for gall-bladder calculi should be performed in the presence of a dilated, tender thick-walled gall-bladder.