We studied the arterial blood supply to the sinus node area in 309 consecutive patients undergoing coronary arteriography. Seventy-nine had had a previous myocardial infarction. In two who developed temporary sinus node dysfunction, the sinus node artery arose from the distal portion of a severely stenotic left circumflex coronary artery, but in 307 of the 309 patients the sinus node arteries themselves were free of atheroma. The sinus node artery arose from the right coronary artery in 182 patients, from the proximal 3 cm in 179, near the origin of the acute marginal in one, and left of the crux in two. In 119 patients it arose from the left circumflex coronary artery, proximally in 87, and in 32 from anywhere throughout its length, running posteriorly as the posterior sinus node artery. Finally, eight patients had two sinus node arteries, one arising from the right coronary artery and one from the left circumflex.