Phototherapy (PT) has been reported to increase bile acid excretion in the bile of adults with liver cirrhosis. We investigated the effect of PT on the levels of serum total bile acids (STBA), conjugated cholic acid (CCA), conjugated chenodeoxycholic acid (CCDCA), conjugated lithocholic acid (CLCA), and sulfolithocho-lylgiycine (SLCG) in 13 neonates with unconjugated nonhemolytic hyperbilirubinemia before and after 12 h of PT. The treatment produced a statistically significant (p < 0.02) reduction in both STB A and CCA levels, whereas no effect on the other fractions was observed. The percentage of reduction was the same for STBA and CCA concentrations, indicating that the effect is related to a specific reduction in CCA levels. The magnitude of the expected decrease in the level of serum bilirubin is not correlated with that in bile acids in individual cases. The data are interpreted as suggesting that PT can affect the metabolism of bile acids by decreasing STBA and CCA levels in neonates through an increase in their biliary excretion associated with the reduced intestinal absorption of CCA occurring in newborns.