Miniaturization of ultrasound probes has made possible endoluminal investigation of small duct systems. We have used a 360 degree transaxial real-time sector-scan imaging system with a field of view of 3-5 cm. It operates at ultra-high frequencies, which allows very high resolution. Ten jaundiced patients, aged 35-73 years, were investigated. Malignant bile-duct obstructive disease was present in eight and benign strictures in the remaining two. In all of them the intrahepatic bile ducts had undergone percutaneous transhepatic cannulation for diagnostic and therapeutic purposes. The resolution capacity has been sufficient to allow studies of the bile duct wall and adjacent tissues, and it has thus been possible to study the papilla of Vater, biliary-enteric anastomoses, and adjoining portions of the pancreatic and cystic ducts. Adjacent vessels have been identified. This preliminary study indicates that the character of strictures and the extent of tumour growth may be evaluated, thereby demonstrating the clinical potential of these miniature transducers. Endoluminal ultrasound evaluation of the bile ducts may also be possible intraoperatively and as a supplement to duodenoscopy.