Data from 34 patients were included in the analysis of this open group comparative study comparing a controlled release theophylline given twice daily with immediate release aminophylline given four times daily. The treatment period was of eight weeks duration. There was no significant difference between treatments in clinical assessments of asthma severity or pulmonary function tests. Similarly there were no significant differences between treatments in diary card assessments of asthma symptoms or PERF. Serum theophylline levels were measured prior to the morning dose of test treatment and 2 or 5 hours later, respectively for patients taking immediate release (IR) or controlled release (CR) preparations, at each clinic visit. There was no significant difference between treatments in serum theophylline levels fluctuations, although the dosing interval (12 hours) was twice as long for CR formulation. Six patients reported unusual symptoms, two in the CR group (headache, gastric discomfort) four in the aminophylline group (three headache, one headache and vomiting).