Forty-two morbidly obese patients underwent cholecystectomy at the time of gastroplasty, primarily for prophylactic reasons. Preoperatively, 37 patients underwent ultrasonography (US) of the gallbladder and oral cholecystography, four US only, and one oral cholecystography only. There was one indeterminate US study (2.4%) and one indeterminate oral cholecystogram (2.7%). At US study, gallstones were detected in six of the eight patients with gallstones (sensitivity = 75%), and the gallbladders of all 32 patients without gallstones were categorized as normal (specificity = 100%). The gallstones not identified measured 1-2 mm in diameter. Oral cholecystographic study enabled detection of gallstones in three of the seven patients with gallstones (sensitivity = 43%) and categorized as normal all 30 gallbladders without gallstones (specificity = 100%). The gallstones not seen were small and included those not detected by US. The results suggest that US is equal or superior to oral cholecystography for detection of cholelithiasis in obese patients.