BACKGROUND Limited information exists on the innervation of human cardiac valves and the relationship of nerve fibers and terminals with functional elements within leaflets. RESULTS We examined human AV and arterial valves, obtained postmortem and at surgery, using quantitative immunohistochemical, histochemical, and confocal microscopic techniques. Significant differences in nerve density and distribution were found both between and within cardiac valves. Nerve density within the anterior leaflet of the mitral valve, for example, was twofold greater than that in the posterior leaflet (P < .001). Nerves within the AV valves were situated in the atrial layer and extended over the proximal and medial portions of the leaflets, whereas those in the arterial valves were situated in the ventricular layer. No nerves reached either the free edge or the fibrous core of the leaflets. The arterial valves displayed a similar density of innervation, except for the noncoronary leaflet of the aortic valve in which the innervation was attenuated (P < .01). The innervation of aortic valvar leaflets was age dependent. Nerve terminal arborizations, arising from myelinated nerves and exhibiting variable morphology, were detected in all four cardiac valves and in some tendinous cords. Nerve terminals exhibited either acetylcholinesterase activity or tyrosine hydroxylase and neuropeptide Y immunoreactivity. Varicose nerve fibers occurred in close physical proximity to valvar endothelial, smooth muscle, and fibroblast cells. CONCLUSIONS Human cardiac valves have distinct patterns of innervation that comprise both primary sensory and autonomic components. The presence of distinct nerve terminals and the close association of varicose nerve fibers with endothelial, smooth muscle, and fibroblast cells suggest a possible neural involvement in the control of valvar function.