There are many causes of left main coronary artery disease, the first of which is atherosclerosis. Other rarer causes may be observed, such as acute and chronic occlusions, spasm and primary and secondary dissection. The prevalence of stenosis of the left main coronary artery at coronary angiography is about 5%. The risk factors are the same as for coronary artery disease. The symptoms are angina, especially unstable angina. The diagnosis is suspected on the finding of an extremely positive exercise stress test, confirmed by coronary angiography. The results of the prospective large scale Veterans Administration trial showed surgery to be the treatment of choice with a 30 months survival of 80% in the surgical group compared with 64% in the medical group. The operative morbidity and mortality is less than 10% at present. Recent studies have reported a medium-term mortality of 4.3 to 10.25% with follow-up periods of 24 and 43 months respectively. The long-term survival and functional improvement are excellent, with values of nearly 80%. Chronic occlusion of the left main stem is rare, 0.01 to 0.7% in coronary angiographic studies. There is no difference in presentation, electrocardiographic or stress test features compared with other severe coronary artery disease. The diagnosis is angiographic and the treatment surgical because of the mediocre natural history with risks of sudden death and severe infarction. Acute occlusion of the left main coronary is rare for generally fatal. The mechanism is acute thrombosis and the clinical presentation is that of extensive infarction usually with cardiogenic shock.(ABSTRACT TRUNCATED AT 250 WORDS)