Theophylline metabolism and elimination during childhood are age-dependent. The total clearance, which is the sum of metabolic and renal clearances is highest in infants and young children. To differentiate between the two pathways, theophylline concentrations were measured in both serum and urine in 16 children aged 2 weeks-16 years using the fluorescence polarization immunoassay. It was found that both partial clearances assume maximal values at about the same age, but the fraction of total clearance attributable to the renal route decreases continuously.