Serum bile acid patterns were determined in 31 infants ranging in age from 4 days to 24 weeks. Fifteen infants with extrahepatic biliary atresia had a mean concentration of serum bile acids of 90 microng/ml +/- 47 SD;; 16 infants with neonatal hepatitis had a mean concentration of 60 +/- 35.5 SD. The chenodeoxycholate/cholate ratio was greater than one in 13 infants (87%) with atresia and in 10 infants (66%) with neonatal hepatitis. Except for the tendency of a higher total concentration of serum bile acids in infants with atresia, a single serum bile acid value does not differentiate neonatal hepatitis from extrahepatic biliary atresia. The high proportion of chenodeoxycholate in extrahepatic atresia is different from the pattern in other types of cholestatic disease and may reflect an underlying hepatitis.