The effect of ozagrel (OKY-046), a selective thromboxane A2 synthetase inhibitor, on theophylline disposition was studied in 12 asthmatic patients. Ozagrel was administered at a dose of 200 mg twice daily for 24 weeks to 4 outpatients receiving oral theophylline medication. Blood samples were drawn from each patient visiting the hospital at one to four-week intervals. There were no clinical significant changes between the time course profiles of serum theophylline concentration during the treatment and those after the cessation of ozagrel dosing. In addition, another 8 hospitalized patients received a single intravenous infusion of aminophylline (200 mg as theophylline) before and after ozagrel treatment of 200 mg twice daily for 7 days. No statistical significant alteration in elimination half-life, total body clearance or volume of distribution of theophylline was observed by the administration of ozagrel. These results suggest that ozagrel does not inhibit the metabolism of theophylline despite having an imidazole ring in its chemical structure.