It is generally agreed that the bronchodilator response to theophylline can be directly related to plasma concentration and that the efficacy of theophylline therapy can be improved if plasma concentrations are monitored. However, it is not known whether this improvement is realised in routine clinical practice. In this study, theophylline plasma concentrations were measured in 359 outpatients attending a teaching hospital respiratory medicine clinic. In nearly three-quarters of these patients (73.2%), the plasma concentration of theophylline was below the recommended therapeutic range of 55–110 μmoles/liter (10–20 mg/liter). These results were made available to the medical staff at the patient consultation. One hundred and seventy-seven patients reattended for a second visit and their plasma concentrations remeasured. At this visit, 63.2% of plasma levels were below the therapeutic range of 55–110 μmoles/liter; the percentage of patients in this therapeutic range had risen from 25.6% at the first visit to 33.9% at the second. Concentrations in excess of 110 μmoles/liter were rare and accounted for only 1.1–2.8% of samples assayed. The increase in the percentage of patients whose plasma concentration was in the therapeutic range may have been due to a better patient compliance resulting from provision of the “on-line” plasma monitoring service. The data suggest that physicians are conservative in their use of theophylline and may be maintaining the patients at a therapeutic range of 28–55 μmoles/liter, particularly when the drug is given with other therapeutic agents such as the β2-adrenoceptor agonists.