In a prospective study in 73 patients with obstructive pulmonary disease, aged 63.5±13.5 years (SD), it was found that theophylline dose, cigarette smoking and age were all significant determinants of the steady-state trough plasma theophylline level during oral administration of the drug. As the predictive efficiency of the three factors combined amounted only to 25%, firm dosage recommendations cannot be made. Even among elderly patients, tobacco smokers had significantly lower plasma concentrations of theophylline. It is suggested that in order to obtain trough plasma concentrations of 50 µmol/l (≈9 µg/ml), a non-smoking 50 year-old patient would require 9.8 mg/kg/day of oral theophylline, the dose increasing to 14.2 mg/kg/day in smokers of the same age. These doses should probably be reduced by 15–20% in 75 year-old patients.