Coronary angiograms were reviewed in 31 patients with idiopathic prolapse of the posterior mitral leaflet. There were 19 males and 12 females, ranging in age from 33 to 69. The coronary artery which supplied the posterior descending branch was designated as dominant. There were 27 dominant right coronary arteries and 4 dominant left coronary arteries. Attention was paid to whether the origin of the vessel which courses in the posterior atrioventricular groove branch was from the right coronary artery or the left circumflex. In the dominant right coronary artery group, the artrioventricular groove branch arose from the right coronary artery alone in 6 and from the left circumflex alone in 1 patient, and in 20 patients, from both. In the dominant left coronary artery group, the atrioventricular groove branch arose from the left coronary artery in all 4 patients. The frequency of dominant right coronary artery and left coronary and the origin of the atrioventricular groove branch did not differ in the patients with prolapse of the mitral valve from a control group of 30 patients similarly analyzed. In all instances, the atrioventricular groove branch arose from either the right coronary artery or the left circumflex, or both. In no case was the atrioventricular groove branch totally absent. The results of this investigation do not support the thesis, previously advanced by others, that prolapse of the mitral valve is related to absence of the left circumflex coronary artery, but indicate a normal range of variation in coronary arterial distribution.