Examination of cardiac vascular endothelium with scanning electron microscopy, and transmission electron microscopy of previously scanned tissue, revealed several regions of the coronary venous system that contained 'cluster' microvilli. These consisted of 2-15 microvillous projections that emanated radially from a common base or were grouped into a fan-like arrangement. Although rare, these clusters, when present, were widely distributed over the endothelial cell surface.