Severe stenosis (greater than 70%) of the main stem of the left coronary artery may rarely be asymptomatic. This particular clinical presentation, which has recently been highlighted, is probably commoner than had previously been thought. Though usually a chance finding, its presence should be suspected when exercise stress tests are strongly positive at low work loads; confirmed at coronary angiography, asymptomatic stenosis of the left main coronary artery is no different from symptomatic stenoses with regard to the extent and severity of the coronary lesions; they do differ, however, in that left ventricular function is preserved and may remain normal over a long period. This has therapeutic implications in that systematic surgery of first intention may not be mandatory in these cases.