The pharmacokinetic behavior of theophylline was determined in 12 patients during an acute exacerbation of their chronic obstructive pulmonary disease. A 5.6 mg/kg loading dose of aminophylilne was administered, followed three hours later by a 0.9 mg/kg/hr continuous infusion. The loading dose increased the serum theophylline level an average of only 5.77 microgram/ml. After the loading dose, only five patients had levels greater than 10 microgram/ml. Mean initial drug clearance was 0.77 L/kg/hr, half-life 9.1 hr, and apparent volume of drug distribution .887 L/kg. Wide inter- and intrapatient pharmacokinetic variability was observed. The variability of drug distribution was inversely correlated with the arterial pH. These patients with chronic obstructive pulmonary disease appeared to require more theophylline when acidemic than when alkalemic to achieve similar serum theophylline concentrations.