PURPOSE: To reestablish the size criterion of the gallbladder on ultrasonography (US) for the differential diagnosis of biliary atresia from neonatal hepatitis. MATERIALS and METHODS: Abdominal US was performed in 201 patients with jaundice and 40 patients without evidence of jaundice or hepatobiliary illness (all with the age less than 4 months). US was performed in fasting (fasting for at least 4 hours) to measure the length of the gallbladder and calculated the area of the gallbladder lumen. The morphology of the gallbladder was classified into three types: normal, elongated and atretic. To evaluate the contractibility of the gallbladder, the length of the gallbladder and area of the gallbladder lumen was again measured 1 hour after feeding. The final diagnosis included biliary atresia in 79 patients and neonatal hepatitis in 83 patients. RESULTS: Differences in the length, area, and morphology of the gallbladder were statistically significant among three groups, the normal group, neonatal hepatitis group and biliary atresia group. (length and area of gallbladder; normal group > neonatal hepatitis > biliary atresia). The differences in the length and area of gallbladder between pre- and postmeal state were statistically significant in the normal and neonatal hepatitis groups whereas those of biliary atresia were not significant (p=.085). When the empirical size criterion of the gallbladder (< 15 mm in length) was applied, the sensitivity, specificity and diagnostic accuracy for the differential diagnosis of biliary atresia from neonatal hepatitis were 52%, 82%, and 67%, respectively. Meanwhile, if the area criterion (< 30 mm2 in area) was applied, the sensitivity, specificity and diagnostic accuracy were 67%, 85%, and 75%, respectively. CONCLUSION: Ultrasonographic evaluation of the morphology as well as size of the gallbladder are helpful in the differential diagnosis of biliary atresia from neonatal hepatitis. Therefore, since the measurement of the area of gallbladder lumen on US reflect both size and morphology of the gallbladder, it can be used for the differential diagnosis.