Calcific aortic valve disease (CAVD) is a leading cardiovascular disorder in the elderly. Diseased aortic valves are characterized by sclerosis (fibrosis) and nodular calcification. Sclerosis, an early pathological change, is caused by aortic valve interstitial cell (AVIC) proliferation and over-production of extracellular matrix (ECM) proteins. However, the mechanism of aortic valve sclerosis remains unclear. Recently, we observed that diseased human aortic valves over-express growth factor neurotrophin 3 (NT3). In the present study, we tested the hypothesis that NT3 is a pro-fibrogenic factor to human AVICs. Methods and Results: AVICs isolated from normal human aortic valves were cultured in M199 growth medium and treated with recombinant human NT3 (0.10 µg/ml). An exposure to NT3 induced AVIC proliferation, up-regulated the production of collagen, MMP-9 and augmented collagen deposition. These changes were abolished by inhibition of the Trk receptors. NT3 induced Akt phosphorylation and increased cyclin D1 protein levels in a Trk receptor-dependent fashion. Inhibition of Akt abrogated the effect of NT3 on cyclin D1 production. Further, inhibition of either Akt or cyclin D1 suppressed NT3-induced cellular proliferation, MMP-9 and collagen production, as well as collagen deposition. Conclusions: NT3 up-regulates cellular proliferation, ECM protein production and collagen deposition in human AVICs. It exerts these effects through the Trk-Akt-cyclin D1 cascade. Thus, NT3 is a pro-fibrogenic mediator in human aortic valve, and over-production of NT3 by aortic valve tissue may contribute to the mechanism of valvular sclerosis.