To elucidate the prevalence and features of painless myocardial ischemia among diabetic patients, 44 consecutive patients with angiographically-documented coronary artery disease and positive treadmill tests were examined. They were 26 with diabetes and 18 without it. Painless myocardial ischemia was defined as the absence of chest pain with 1 mm or more ST segment depression during the exercise stress tests. The severity of ischemia was determined by the magnitude of the ST segment depression. Painless myocardial ischemia was observed in 18 of the 26 (69%) diabetics, and in three of the 18 (17%) non-diabetics (p less than 0.005). The frequency of painless ischemia in the diabetics was relatively high regardless of the severity of ischemia, while painless ischemia was less frequent in the non-diabetics with severe ischemia. With a level of 2.5 mm ST depression, 11 of 12 (92%) diabetics were free of pain compared to four of 11 (36%) non-diabetics (p less than 0.01). Absence of chest pain during the exercise tests was not concordant with prior angina in diabetics, as opposed to non-diabetics in whom both clinical and exercise-induced angina developed concordantly. The diabetic patients without chest pain had a higher prevalence of three major diabetic complications such as neuropathy, nephropathy and retinopathy compared to those developing chest pain (p less than 0.025). It was concluded that in diabetics, painless myocardial ischemia is frequently observed during exercise stress tests and its prevalence is relatively high regardless of the severity of ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)