Early diagnosis of extrahepatic biliary atresia (EHBA) is very important for a successful bile drainage procedure. An urgent and extensive investigation is warranted to make a differential diagnosis of EHBA from other cholestatic disease, especially neonatal hepatitis (NH). The diagnosis may be made by laparotomy with cholangiogram and liver biopsy but these procedure are invasive. Because a cone-shaped fibrous tissue was always found at the porta hepatis during Kasai's procedure and that type of fibrous tissue has never been seen in other types of cholestatic jaundice, the authors have used ultrasonography (US) to detect fibrous remnants at the porta hepatis in infants clinically suspected of having EHBA and identified a triangular- or tubular-shaped echogenic density in the bifurcation of the portal vein at the porta hepatis in EHBA and called it an ultrasonographic "triangular cord (TC)". The authors determined that the presence of TC denoted the EHBA. On the other hand, nonvisualization of TC was interpreted as consistent with NH. In the present study, the authors reviewed ultrasonographic examination and hepatobiliary scintigraphy in a series of 41 infants suspected of having EHBA or NH. The TC was identified in thirteen infants. In twelve of thirteen infants with TC found by US, the diagnosis of EHBA was confirmed at the time of Kasai's Roux-en-Y hepatoportojejunostomy operation. The remaining one died at 15 months of age without having treatment. TC was not visualized in 28 infants. Twenty-seven of 28 infants with absent TC improved clinically through medical treatment for NH. The other, diagnosed to have NH by needle and wedge liver biopsies, eventually showed a TC on follow-up US examination performed 40 days after the initial examinations. Moreover, follow-up percutaneous liver biopsy specimen showed typical findings of biliary atresia with bile duct proliferation and portal fibrosis. The patient underwent a Kasai hepatoportoenterostomy. On review of the hepatobiliary scintigraphy, all 13 infants with TC had no intestinal excretion of isotope. Thirteen of 28 infants (46%) without TC also had no intestinal excretion of isotope in the 24hour follow-up but all of them were confirmed to have NH by percutaneous liver biopsy and their subsequent clinical course confirmed NH with the detection of bile in their stool except one. On the basis of these results, the authors conclude that ultrasonographic TC is a very specific finding representing the fibrous cone at the porta hepatis and is a quick, simple, and definitive tool in the noninvasive diagnosis of EHBA. If TC is visualized, no further studies are necessary and exploratory-laparotomy can be done. If TC is not visualized, hepatobiliary scintigraphy is recommended to demonstrate bile duct patency. Percutaneous liver biopsy is only required if neither TC nor intestinal excretion of isotope is seen. However, early exploration or close US follow-up study is recommended in any patient suspected of having EHBA clinically, even if liver biopsy confirms the presence of NH.