The coronary arteries of five hearts of patients known to have had Marfan's syndrome were studied microscopically. In four of the five cases the extramural coronary arteries showed structural changes consisting of accumulations of acid mucopolysaccharides and a concomitant disruption of the connective tissue elements. These changes closely resembled the changes that frequently occur in the aorta and that are generally described as "cystic medial necrosis". They are considered to be an important expression of wall weakening. In one case a laceration, probably traumatic, of a coronary artery led to formation of an "incomplete dissecting aneurysm." "Cystic medial necrosis" was found once in the proximal segment of the sinus nodal artery and once in the musculoelastic layer of a coronary artery that reached the atrioventricular node from the ventricular septum by perforating the anulus fibrosus. However, in contrast to findings in a previous report, the intramural coronary arteries were otherwise unaffected. The clinical significance of these data is as yet unknown particularly since these alterations are intramural and initially do not cause narrowing of the lumen. Nevertheless, the possibility of coronary arterial involvement by the metabolic disorder of Marfan's syndrome should always be considered during clinical assessment of this lesion.