Gathering data on the prognosis, detection, and natural history of asymptomatic left main coronary artery disease with silent myocardial ischemia is difficult. Epidemiologic studies of unexpected death and postmortem studies on silent myocardial infarction suggest this entity to be common. We reviewed 89 consecutive patients with left main coronary artery disease (LMD), defined as 50% or greater reduction of luminal diameter. Of this group, 10 patients (11%) were asymptomatic (ALMD) and 79 patients (88%) were symptomatic (SLMD). All 10 ALMD patients were men, with a mean age of 53 years (range 40 to 65). Treadmill tests (TMT) were performed for: ECG abnormalities six; pre-jogging evaluation two; risk factor evaluation two. The TMT within 9 minutes showed 2 mm or greater ST depression in seven (70%) and 1 to 2 mm in three (30%). Similar TMT results were obtained in the SLMD group, although two patients had negative responses. The degree of stenosis of the left main coronary artery and the frequency of three-vessel disease were similar in both groups. The ejection fraction (EF) and contractile pattern of the left ventricle (LV) were normal in all 10 ALMD patients, but the left ventricular end-diastolic pressure (LVEDP) was abnormal in three (70%). In the SLMD group, 51 (64%) had an abnormal ejection fraction, 50 (65%) had wall motion abnormalities, and 25 (32%) had an abnormal LVEDP.