Diabetes mellitus, a disease with a wide prevalence, has major cardiovascular effects, being a risk factor for the development of ischemic heart disease and congestive heart failure. The aim of this open, multicenter study was to assess the antiischemic efficacy and tolerability of trimetazidine, a metabolic agent acting at the myocardial mitochondrial level, in diabetic patients with stable effort angina treated previously with a single conventional antianginal drug. Fifty diabetic patients (mean age 58 years) with proven coronary artery disease, stable effort angina for at least 3 months, and positive, comparable results of two initial treadmill exercise tests separated by a 1-week interval were included in the study. They continued their conventional antianginal monotherapy with a long-acting nitrate, beta-blocker, or calcium channel blocker. After stabilization, 4-week therapy with trimetazidine, three times daily, 20 mg was initiated in combination with previous treatment. The results showed a significant improvement in exercise tolerance (440.2 vs. 383.2 s; P < 0.01), time to 1-mm ST-segment depression (358.3 vs. 301.6 s; P < 0.01), time to onset of anginal pain (400.0 vs. 238.3 s; P < 0.01), and total work (9.39 vs. 8.67 metabolic equivalents, P < 0.01). Maximal ST-segment depression was attenuated compared with baseline (1.82 vs. 1.91 mm). Other findings included a significant decrease in the mean frequency of anginal episodes (3.06 vs. 4.79 per week; P < 0.01) and in mean nitrate consumption (2.29 vs. 4.2 doses/week). These results suggest that trimetazidine may be effective and is well tolerated as combination therapy for diabetic coronary artery disease patients uncontrolled with a single hemodynamic agent.