Pentoxifylline, a methyl xanthine derivative, improves symptoms of peripheral vascular disease probably by reducing whole blood viscosity. The authors assessed the value of this agent in treating myocardial ischemia in 11 patients with angiographically documented coronary artery disease and stable angina pectoris. Maximal, symptom limited treadmill exercise stress tests were performed before and after six weeks of therapy with 1200 mg of pentoxifylline per day. Clinical symptoms proved in 9 [82%] of patients; none developed drug side effects. After therapy, mean total exercise time [7.7 +/- 1.3 vs 10.1 +/- 1.2 minutes], time to onset of angina [5.5 +/- 0.9 vs 7.9 +/- 1.0 minutes], heart rate at onset of angina [93.4 +/- 6.7 vs 112.0 +/- 10.5 beats/min] and rate at onset of ST depression [94.0 +/- 5.8 vs 115.9 +/- 7.4 beats/min] all increased significantly [p less than 0.05]. Mean maximum ST segment depression also decreased [1.6 +/- 0.3 vs 1.2 +/- 0.4mm], but the difference was not significant. Thus, pentoxifylline increases exercise performance in patients with angina pectoris and increases exercise capacity before development of of myocardial ischemia. It may, therefore, be a useful agent for treating ischemic heart disease.