The relation between the length of the main left coronary artery and the degree of atherosclerosis in its branches was studied by postmortem examination in 204 subjects aged 20 to 90 years. The findings suggest that in cases with a short main left coronary artery the atherosclerotic lesions in the anterior descending and circumflex branches appear earlier, progress faster at higher levels of severity, and lead more frequently to myocardial infarction, than in cases with a long left coronary artery trunk. In cases over the age of 50 years, where disease is expected to have developed, it was shown that the degree of atherosclerosis in the left anterior descending and circumflex branches was inversely related to the length of the main left coronary artery. The correlation coefficients were -0-527 and -0-428, respectively, and in either case a test for zero correlations was significant (P less than 0-001). The possible changes in the haemodynamic and mechanical conditions associated with the variations of the anatomical pattern of the coronary arteries and their influence in the development of atherosclerosis are discussed. It is suggested that the length of the main left coronary artery is a congenital anatomical and possibly hereditary factor influencing the rate of development of atherosclerosis in the branches of the main left coronary artery.