The positions of the coronary arterial ostia were examined in the cadavers of 38 adults. All the ostia were related to the aorta and none to the pulmonary artery. The aortic valves in 37 specimens were normal and possessed three cusps, while one was bicuspid. In the normal specimens, the main right and left coronary ostia were confined to the anterior and left posterior aortic sinuses respectively, apart from one specimen in which the right ostium was related to the right posterior sinus. Six specimens had small accessory ostia, close to the main right ostium. The majority of ostia were situated on or immediately below the supravalvular ridge. Circumferentially, the ostia clustered near the maximum curvature of the sinus. In the single specimen where the aortic valve was bicuspid, both ostia lay more-or-less symmetrically in the sinus above an anterior cusp which showed evidence of formation from two components. The reasons for confinement of the coronary ostia two of the three aortic sinuses are not clear. Microscopic examination of serial sections through 22 human embryos of 5.0-17.5 mm CR length (Carnegie stages XIII-XIX) confirmed that the earliest vessels in the heart wall develop subepicardially near the apex at stage XV. The network extends centripetally and only at stage XVII could coronary arterial stems, communicating with the aortic lumen, be identified. The sequence suggests that confinement of the coronary ostia to the interior and left posterior sinuses probably occurs because these represent the most accessible contact points for the centripetally growing vascular plexus.