Urine samples from 92 individuals of a healthy pediatric population ranging in age from less than 24 h to 12 years were analyzed for organic acid content (amino acids excepted). A few samples from adults were included for purposes of comparison. Acids were extracted by using an anion-exchange procedure, converted to trimethylsilyloxime derivatives, and analyzed by gas chromatography with the help of a small computer system. Identifications were confirmed by using a combined gas chromatograph/mass spectrometer/computer system. The data has been tabulated according to five pediatric age groups. Despite large individual variation within age groups, significant differences in amounts of several acids excreted (based on urinary creatinine concentration) have been determined between age groups. Such differences might be due to one or a combination of several factors, including changes with age in creatinine excretion, in physiologic maturity, and in diet. Such data may help to provide a basis for diagnosing metabolic abnormalities in pediatric patients.