A prospective study was designed to assess the accuracy of gray scale ultrasonography in the evaluation of cholelithiasis. A series of 111 patients with nonvisualization on first-day oral cholecystography underwent ultrasonic cholecystography, and the results were compared with subsequent second-day visualitzation or surgical findings. In 75 cases sufficient data were present to assess the accuracy of the method. Ultrasound correctly diagnosed 68 (91%) of these cases with regard to gallstones. Only 6 gallbladders could not be visualized ultrasonically and all subsequently proved to have gallstones. It is concluded that improved ultrasonic technique is of considerable value in investigating the nonvisualized gallbladder. Valuable information conderning other upper abdominal pathology was often discovered.