In a prospective study, 37 patients (20 women, 17 men, mean age 75 [41-93] years) with extrahepatic obstructive jaundice were investigated, within 24 hours of admission, by transcutaneous ultrasound (US), endoscopic ultrasound (EUS) and endoscopic retrograde cholangiopancreatography (ERCP). EUS was always performed after US and immediately before ERCP, and the findings were recorded without any knowledge of the results of ERCP (the 'gold standard'). Dilatation of the common bile duct was demonstrated by all three methods. Concretions in the distal common bile duct causing obstructive jaundice were demonstrated by EUS in 15 out of 16 cases, but in only seven cases by US. All 21 cases with an underlying malignant cause were correctly diagnosed by EUS and ERCP, and the level of the biliary obstruction was accurately determined; using US this was possible in only 15 (70%) and 18 (85%) cases. In comparison with ERCP, which imaged the tumour only indirectly, EUS showed the tumour itself and allowed local and regional staging in all cases. EUS is superior to US for elucidating the cause of biliary outflow obstruction and allows reliable local and regional staging. It is of additional benefit in deciding on appropriate therapy. A disadvantage is the impossibility at present of undertaking surgical therapy during EUS investigations.