While monitoring oral aminophylline (theophylline ethylenediamine) therapy, larger doses (milligrams per kilogram) were necessary for children than for adults to achieve similar therapeutic serum levels. The mean apparent theophylline clearance (ATC) determined in 59 children was significantly higher (P less than .001) than determined in 114 adults (84.8 versus 51.4 ml/hr/kg). Additionally, the younger children (1 to 9 years) eliminated theophylline more rapidly than the older children (0 to 18 years) (93.8 versus 77.3 ml/hr/kd). When initiating continuous oral aminophylline therapy, dosage guidelines, based on the mean ATC calculated for each group, were predicted, and trough theophylline serum levels of 10mug/ml were obtained. The guidelines for aminophylline were 28.0, 23.2, and 15.6 mg/kg/day administered in four divided doses for the younger children, older children, and adults, respectively. Monitoring of serum levels is suggested to further individualize the patient's aminophyline therapy.