Intravenous aminophylline was administered to 13 subjects (6 normal, 7 with chronic obstructive pulmonary disease), and multiple blood specimens were drawn over an 8-hour period for theophylline analysis. Half of the samples were obtained during the distribution phase of the drug and the remainder during the elimination phase. These data were entered into a computer program that both calculates and graphically displays individual two-compartment pharmacokinetic data, and recommends a dosing regimen. Analysis of these data demonstrates wide variability in the theophylline volume of distribution, half-life, and predicted dosage regimen. Dosage regimens can be individualized by obtaining two specimens for theophylline analysis during the elimination phase after intravenous administration of the drug; these regimens correlate extremely well (r2 = 0.95) with those designed using all the data points.