Our aim was to evaluate the efficacy of ultrasonographic (US) examination in the pre-operative diagnosis of biliary atresia (BA) with special reference to the presence or absence of extrahepatic bile duct. Thirty consecutive neonates and infants aged 8 to 169 days (mean: 62 days) suspected of having biliary atresia were examined pre-operatively in real time B-mode ultrasonography. We used a 5 or 7.5 MHz probe of micro convex type. Patients were fasted and sedatives administered. When the common bile duct was absent, we considered it a positive finding for BA diagnosis; if not, it was considered a negative finding. A definitive diagnosis of BA was confirmed at surgery by gross morphology or intra-operative cholangiography. US findings had a sensitivity of 83% (19 of 23 BA patients), a specificity of 71% (5% of 7 non BA patients) and an accuracy rate of 80%. The positive predictive value was 90% (19 of 21), while the negative predictive value was 56% (5 of 9). There were four false-negative cases. Two were BA cases with patent distal common bile duct, one was BA in which the hepatic artery was determined to be the common bile duct, and the other was a subtype of extrahepatic bile duct dilatation (the so-called, "correctable type"). We employed US criteria for visualization of the extrahepatic bile duct for pre-operative diagnosis of BA. US examination referring to the presence or absence of the extrahepatic bile duct is an effective and useful method for clinical survey.