The usual ultrasound findings in acute cholecystitis include diffuse hyperreflective wall thickening, hazy wall delineation and gallbladder distension. In 11 patients with acute cholecystitis, an additional sign was seen consisting of a hyporeflective or sonolucent layer, continuous or interrupted, within the hyperreflective, thickened gallbladder wall. Anatomic-pathologic correlation, comparative CT [computed tomography] scans and clinical follow-up studies showed subserosal edema and necrosis to be the most probable cause of this finding.