Ten men and 10 women out-patients who were attending a chest diseases clinic and who received an oral theophylline preparation were studied to determine the effectiveness of the use of serum theophylline assays on the monitoring of their treatment. Criteria were laid down with regard to the indications for drawing blood samples, interpretation of the results and documentation. There were definite indications for drawing samples in nine patients, no indications in three patients, and relative contraindications in eight patients. There were inadequate records for 14 patients. Adequate records permitting full interpretation of the results were kept for six patients. The therapeutic range was not stated in the assay report from the laboratory. The daily theophylline dose was stated incorrectly for seven patients. The reports did not determine whether doses were stated as theophylline or aminophylline. Interpretation of the assay results was appropriate for nine patients and poor for five. Reports of the results were sent to 10 of the patients' general practitioners. Four of these letters stated the result in numbers. The letters were sent 1-2 weeks after the blood sample was taken. A number of recommendations are made as a result of this study. The use of serum theophylline assays could be improved.