Fifty-five patients with suspected obstructive jaundice were studied with both ERCP and abdominal ultrasound. Biliary tract obstruction was documented in 49 patients and parenchymal liver disease in 7. A definite cause of jaundice was demonstrated by ERCP in 45 of 49 patients with biliary tract obstruction but in only 28 of 49 patients by ultrasound. Bile duct dilatation, where present, was detected by ERCP in only 33 of 42 patients. ERCP was particularly effective in patients with common duct stones; common duct calculi and a dilated common duct were detected in 15 of 16 such patients. Dilated bile ducts where present were detected by ultrasound in 22 of 42 patients. Ultrasound was particularly helpful in patients with pancreatic cancer in whom a dilated common duct could not be opacified during ERCP. ERCP and abdominal ultrasound together provide a rapid and safe method of diagnosis in the jaundiced patient with suspected bile duct obstruction.