To clarify the relationship between hyperbilirubinaemia and abnormal results of biochemical liver function tests in infants with breast milk jaundice (BMJ), 58 breast-fed infants with indirect hyperbilirubinaemia were enrolled in this study. Sera obtained from the above infants were subjected to routine liver function tests. Although serum transaminases were within normal limits in all 58 patients, serum alkaline phosphatase levels were abnormally increased in 13, gamma-glutamyltranspeptidase in 8 and total bile acids in 11 out of all patients examined. A total of 18 (31%) patients had abnormal results in at least one item of the liver function tests. The intrinsic bile acid loading test showed postprandial increases in bile acids in 5 of 16 (31%) patients examined at either 60 or 120 min, while all 13 breast-fed, age-matched controls had no abnormal results. The decrease in rate of serum bilirubin levels after the 3-day discontinuation of breast-feeding was significantly less in patients with increased fasting bile acids than in patients with normal fasting levels of serum bile acids. These results may suggest that mild hepatic dysfunction or cholestasis is associated with indirect hyperbilirubinaemia in some infants with BMJ.