The highest concentrations of epidermal growth factor (EGF) are found in urine, but the physiological role of urinary EGF is unknown. We studied human urinary EGF excretion, by measuring its concentration with a specific homologous RIA, in 265 healthy children from birth until age 16 yr. The absolute concentrations varied widely between individuals. Mean values were approximately 10 ng/ml in 1- to 30-day-old infants; 2.5-fold higher values were found in infants aged 2 to 12 months. During the second year there was a further rise to about 70 ng/ml, and urinary EGF excretion was in the same range in older subjects. The EGF/creatinine concentration ratio was less variable. The mean ratio increased 6-fold from birth to the second year of life. Thereafter, the EGF/creatinine ratio decreased gradually to one-third of the peak level at puberty. No sex difference was found.