—Abnormal proliferation of vascular smooth muscle cells (VSMCs) is an important feature of atherosclerosis, restenosis, and hypertension. Although multiple mediators of VSMC growth have been identified, few effective pharmacological tools have been developed to limit such growth. Recent evidence indicating an important role for oxidative stress in cell growth led us to investigate the potential role of aldose reductase (AR) in the proliferation of VSMCs. Because AR catalyzes the reduction of mitogenic aldehydes derived from lipid peroxidation, we hypothesized that it might be a potential regulator of redox changes that accompany VSMC growth. Herein we report several lines of evidence suggesting that AR facilitates/mediates VSMC growth. Stimulation of human aortic SMCs in culture with mitogenic concentrations of serum, thrombin, basic fibroblast growth factor, and the lipid peroxidation product 4-hydroxy-trans-2-nonenal (HNE) led to a 2- to 4-fold increase in the steady-state levels of AR mRNA, a 4- to 7-fold increase in AR protein, and a 2- to 3-fold increase in its catalytic activity. Inhibition of the enzyme by sorbinil or tolrestat diminished mitogen-induced DNA synthesis and cell proliferation. In parallel experiments, the extent of reduction of the glutathione conjugate of HNE to glutathionyl-1,4-dihydroxynonene in HNE-exposed VSMCs was decreased by serum starvation or sorbinil. Immunohistochemical staining of cross sections from balloon-injured rat carotid arteries showed increased expression of AR protein associated with the neointima. The media of injured or uninjured arteries demonstrated no significant staining. Compared with untreated animals, rats fed sorbinil (40 mg · kg⁻¹ · d⁻¹) displayed a 51% and a 58% reduction in the ratio of neointima to the media at 10 and 21 days, respectively, after balloon injury. Taken together, these findings suggest that AR is upregulated during growth and that this upregulation facilitates growth by enhancing the metabolism of secondary products of reactive oxygen species.