A clinical trial of allopurinol (Zyloric) for myocardial protection. This study explored myocardial protective effects of allopurinol at various doses. Ninety patients undergoing coronary artery bypass or repair or replacement of cardiac valves were divided into three groups of 30 patients each in accordance with the amount of allopurinol administered to patients in each group. Patients in group I received no allopurinol, those in group II received low-dose allopurinol (total dose 1200 mg), and those in group III received high-dose allopurinol (total dose 2400 mg). Aspartate aminotransferase, cardiac isoenzyme of creatine kinase, and lactic dehydrogenase levels were measured up to 5 days after operation. Concentrations of allopurinol and oxypurinol were also measured before initiation of cardiopulmonary bypass and at the start and at the end of aortic crossclamping. Postoperative aspartate aminotransferase, creatine kinase, and lactate dehydrogenase 1 plus lactate dehydrogenase 2 levels in group III were significantly lower than those in groups I and II. Aspartate aminotransferase, creatine kinase, and lactate dehydrogenase 1 plus lactate dehydrogenase 2 levels in group II were lower than those in group I, without statistically significant differences. Plasma oxypurinol concentrations were significantly higher in group III than in group II. It was concluded that allopurinol had resultant high myocardial protective effects in dose-related fashion, but its effect might be attributed to oxypurinol levels formed by its degradation. 