This paper describes a system that permits, for the first time, the in vivo determination of local velocity and endothelial shear stress in the major human coronary arteries. The purpose of the system is to facilitate the study of plaque growth and the relationships between local hemodynamic factors and atherogenesis and restenosis. The three-dimensional anatomy of a segment of the right coronary artery was determined immediately after directional atherectomy via a combination of intracoronary ultrasound and biplane angiography. The highly irregular geometry of the segment was then represented in curvilinear coordinates and a computational fluid dynamics technique was used to investigate the detailed, intravascular velocity profile and shear stress distribution. We found minor flow reversals, significant swirling and a large variation of local velocity and shear stress, both axially and circumferentially, within the artery, even in the absence of significant luminal obstruction. The flow phenomena exhibit characteristics consistent with the focal nature of atherogenesis and restenosis. It is concluded that the technology now exists to determine luminal geometry and local variations in flow fields and endothelial shear stress, in vivo.