A procedure was devised to quickly and reliably determine the patency of the cystic duct in patients suspected of having acute cholecystitis. First the gallbladder was stimulated to empty by a cholecystokinin injection. Thirty minutes later a radiolabeled biliary marker, either 150 muCi 131-I rose bengal or 2 mCi 99-mTc dihydrothioctic acid, was injected, and the accumulation of radioactivity in the liver and gallbladder regions was monitored by external gamma emission imaging and recording devices. The images of diagnostic importance were obtained between 60 and 90 minutes after injection of the tracer. Thirty-nine patients with acute abdominal pain were studied. Ten patients who had acute cholecystitis failed to show gallbladder accumulation of radioactivity, reflecting the cystic duct obstruction that initiates this disease. Twenty-nine patients having a variety of other diseases all showed gallbladder accumulation of activity, indicating in each patient that the cystic duct was patent. No significant adverse effects were noted. We conclude that the procedure is a useful adjunct to the clinical and roentgenographic evaluation of patients with acute abdominal pain.