This multicenter, randomized, investigator-blinded, parallel group study compared the effects of converting patients from a q12h extended-release theophylline preparation (Theo-Dur) to a q24h extended-release product (Uni-Dur). Patients (n = 133) first received open-label Theo-Dur treatment with dosage titrated to achieve peak serum theophylline concentrations of 10-20 micrograms/ml. Patients then were randomized to continue Theo-Dur (n = 64) or to convert to Uni-Dur (n = 60) with peak serum theophylline concentrations maintained in the desired range. Pulmonary function tests were performed during the open-label and blinded periods; patients maintained diaries and performed peak flow measurements before each dose of study treatment. Adverse events were recorded throughout the study. Respiratory status during blinded treatment was rated as the same or improved compared with open-label treatment by > 87% of evaluable patients and physicians, regardless of treatment group. There were no significant differences in mean peak serum theophylline concentrations at baseline, at the final evaluation, or at any point during the study. Few dosage adjustments were necessary (5/52, Uni-Dur; 9/57, Theo-Dur). There were no significant changes in pulmonary function test results or patient diary entries between the open-label and blinded periods. Headache and nausea were the most commonly reported adverse events. In conclusion, converting patients from twice- to once-daily theophylline treatment resulted in no significant changes in any measures of pulmonary function, and there were no significant differences between the groups during the blinded treatment period.