The relation between age and sex and the concentration of bilirubin in serum was evaluated in 6740 men and 11 215 women, ages 13 to 96 years. Mean serum bilirubin concentrations in the men significantly exceeded values in the women over all age groups examined. Further, mean serum bilirubin concentrations were greatest both in males and females in the 19-24 years age group and then declined to former values, which persisted throughout life. Pearson correlation coefficients for bilirubin with liver function indices (albumin and total protein) and with hemoglobin were low in all ages and in both sexes, suggesting that bilirubin concentrations do not correlate with those liver functions not directly concerned with bile pigment processing.