Twenty-two patients with reversible airflow obstruction were entered into a double-blind crossover trial of Phyllocontin Continus tablets and placebo to assess the additional benefit of the controlled-release aminophylline preparation when added to the patients' medication. Most patients were taking inhaled or oral steroids in addition to inhaled beta-sympathomimetics throughout the trial. The dosage of Phyllocontin tablets was adjusted in a pre-trial study to provide serum theophylline levels in the accepted therapeutic range of 10--20 mg/l. In the seventeen patients who completed the 8-week study a small, but significant, increase in both the patients' own recordings of PEFR and weekly clinic measurements of FEV1 was seen whilst on the active drug. Ten patients had improvement in FEV1 greater than 200 ml. Eleven patients recorded subjective improvement on the active drug and five out of ten patients who used inhaled beta-sympathomimetics p.r.n. showed a reduction in their usage of greater than 10%. Side-effects were reported by twelve patients.