We evaluated the diagnostic accuracy of 99mTc-DISIDA scintigraphy as a mean of differentianting biliary tresia from neonatal hepatitis. Tc-DISIDA scintigraphy was visually interpreted by assessing the presence or absence of radioactivity in the intestine or gall bladder. In patients without intestinal radioactivity, we measured the hepatic retention index and the hepatic uptake index. The hepat,ic retention index was expressed as the amount of change of liver activity from 5 minutes to 30 minutes postinjection. The hepatic uptake index was graded visually with 5 minute images using the following scoring scheme :grade 0(normal hepatic uptake), grade 1(decreased hepatic up take), grade 2(hepatic uptake equal to cardiac uptake), and grade 3(hepatic uptake less than cardiac uptake). Age, total bilirubin, and hepatic uptake index were compared between the biliary atresia and the neonatal hepatitis group, between neonatal hepatitis patients with and without intestinal radioactivity, and between the biliary atresia and neonatal hepatitis patients with absent int,estinal radioactivity. The results were as follows : l) None of the 30 hiliary atresia patients showed intestinal radioactivity, while 31/40 neonatal hepatitis patients showed intestinal radioactivity, The sensitivity, specificity, and accuracy of the presence of inlestinal radioactivity .or the diagnosis of biliary atresia was 100%, 78%, and 87%, respectively. 2) In patienis with absent intestinal radioactivity the mean hepatic retention index was 1.5+0.6 in the 16 biliary atresia patient,s, and 1.1+0.2 in the 7 neonatal hepatitis patients(p<0.01). All 7 patients with hepatic retention index over 1.5 had biliary atresia. But there were 9 patients with biliary at,resia below 1.5. 3) No significant differences were found in age, total bilirubin, or hepatic uptake index between biliary atresia and neonatal hepatit.is patients. However t.here were differences in age, total bilirubin, and hepatic uptake index bet.ween neonatal hepatitis patients with and without intestinal radioactivity. The hepatic upt,ake index was significantly lower, age was old, and total bilirubin was low in the group with intestinal radioact,ivity compared the group without intestinal radioactivity(p<0.05). Relation between total bilirubin and the hepatic uptake index was that total bilirubin was relatively low at normal hepatic uptake index in biliary atresia and neonatal hepatitis patients. 4) When hepatic uptake index and hepatic retention index were high it suggest that biliary atresia is more likely, considered relation between hepatic uptake index and the hepatic retention index. Thus, we conclude that Tc DISIDA scintigraphy is accurate in the differential diagnosis of biliary atresia and neonatal hepatitis. In patients without intestinal radioactivity, the hepatic retention index and hepatic uptake index, along with the patients age and total bilirubin level may supplement diagnosis and improve diagnostic accuracy.