Cortisol was determined in the sera of 687 normal, healthy probands between the ages of 5 days and 18 years (neonates, infants, children and adolescents), using a fluorescence polarization immunoassay (FPIA) on the TDx from Abbott Laboratories. The proband collective was divided into 9 age groups and each group into males and females. In accordance with the recommendations of the International Federation of Clinical Chemistry, the 95% scatter range was taken as the reference range. Only a few reference groups showed a normal Gaussian distribution. In addition to the 50th percentile, the 2.5th and 97.5th percentiles were determined for all reference groups. Minimal and maximal values were also determined. The U-test of Mann & Whitney was used to test for significant differences between the individual reference groups. Groups showing no significant differences were combined, and reference ranges were finally calculated for cortisol in the serum of normal, healthy neonates, infants, children and adolescents. The serum cortisol concentration showed a significant increase with age. In the 14-15 years age group, there was also a significant difference between males and females.