Radionuclide cholescintigraphy (RC) is a useful adjunctive diagnostic tool for the identification of acute cholecystitis. False-positive rates, that is, nonvisualization, of 10 to 38 per cent have been reported in patients with factors associated with nonfilling of the gallbladder, such as prolonged fasting and the administration of total parenteral nutrition, pancreatitis, alcoholism or other critical illnesses. The administration of morphine sulfate increases resting pressure of the common bile duct because of constriction of the sphincter of Oddi, and increases the likelihood of gallbladder visualization. We administered morphine sulfate (0.05 to 0.1 milligram per kilogram given intravenously) to 68 patients (including 25 critically ill patients) suspected of having biliary sepsis and who demonstrated nonvisualization of the gallbladder by RC at 30 to 60 minutes. Visualization of the gallbladder occurred within 60 minutes after the administration of morphine sulfate in 38 patients and within 30 minutes in 36 of the 38, aiding in exclusion of the diagnosis of acute cholecystitis in 37 patients. Acute cholecystitis was confirmed by laparotomy in 28 of the remaining 31 patients. There were two false-positive and one false-negative scans, yielding a sensitivity rate of 97 per cent, a specificity rate of 95 per cent, positive and negative predictive values of 0.93 and 0.97, and an accuracy of 96 per cent for this investigative procedure. We conclude that administration of morphine sulfate in conjunction with RC in seriously ill patients enhances the reliability of this test.