A total of 75 jaundiced infants with gestational ages ranging from 37 to 42 weeks were studied during the first 10 days of age to evaluate renal function by measuring endogenous creatinine clearance (Ccr), fractional excretion of N-acetyl-beta-D-glucosaminidase (NAG) to creatinine, fractional excretion of sodium (FENa) and urine osmolality. All jaundiced infants were divided into two groups. Group 1 infants (n = 35) had total serum bilirubin levels ranging between 21 and 39.6 mg/dl (mean 27.2). Exchange transfusions were performed in all group 1 infants at the time of the initial study. Group 2 infants (n = 40), whose total serum bilirubin levels ranged between 12.3 and 20 mg/dl (mean 16.4), received phototherapy, except for 2. Conjugated bilirubin levels were less than 1.0 mg/dl in all these infants. Results were compared with 25 untreated control infants with corresponding gestational and postnatal ages. Follow-up studies were done in 27 of the 35 group 1 infants and in 32 of the 40 group 2 infants prior to hospital discharge, when total serum bilirubin levels had decreased to less than 10 mg/dl. Ccr, fractional excretion of NAG to creatinine, FENa and urine osmolality of group 1 infants were statistically significantly different when compared to those of group 2 and the control infants. The measured parameters in the post-treatment follow-up study of group 1 infants returned to near-normal levels when total serum bilirubin levels became normal. However, no significant differences were seen between group 2 and the control infants in any of the measured parameters.(ABSTRACT TRUNCATED AT 250 WORDS)