Requirements for regular-release oral theophylline were determined in 34 pediatric patients aged 0.25–14.8 years using orally derived pharmacokinetic data and the first-order absorption equation. Large ranges were found in the half-life (2.3–21.3 h) and calculated apparent volume of distribution (Vd) (0.300–1.54 L/kg). Mean serum theophylline concentration was more closely correlated with actual mean concentration (r= 0.61, p < 0.0002) when calculated with the individual patients'Vdvalues than with the standardVdof 0.5 L/kg (r= 0.31, NS), and predictions obtained with the individuals'Vdvalues were more precise (p < 0.05) and less biased than those obtained with the standardVd. The precision of prediction based on individualVdvalues was quantitatively similar (root mean squared error = 3.38 mg/L) to reported predictions based on pharmacokinetic values derived from an initial intravenous course of aminophylline therapy. We conclude that theophylline dosage requirements can be accurately estimated from orally derived pharmacokinetic data and that the method described may be useful for patients in whom intravenous therapy is not required or contraindicated.