To examine the proliferative effect of of raloxifene on human umbilical-vein endothelial cells (HUVECs), and to investigate whether there is an associated increased expression of some key regulators of the cell cycle.Cell culture for different incubation times.University research laboratory.Sources of HUVECs.Measurement of cell proliferation, of protein levels of cyclin A, cyclin B1, cyclin D1, cyclin-dependent protein kinase (CDK) 2, CDK4, and p27(Kip1), and of messenger RNA expression of cyclin A, cyclin B1, and p27(Kip1).Cell proliferation was measured by the 3-(4.5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, the bromo-2'-deoxyuridine assay, and the sodium 3'-[1-(phenylaminocarbonyl)-3,4-tetrazolium]-bis (4-methoxy-6-nitro) benzene sulfonic acid hydrate assay. Changes in protein expression were measured by immunoblotting, and modifications in gene expression were measured by quantitative real-time polymerase chain reaction.Both 1 nM and 10 nM of either E(2) or raloxifene achieved a similar increase in cell proliferation. The pure antiestrogen ICI 182780 only blocked the E(2)-induced proliferative effect. Western blot experiments detected an increase in the expression of only cyclin A and B1, and a decreasing trend for p27(Kip1). Enhancements in gene expression were observed in response to E(2) and raloxifene for cyclin A and B1. No significant changes were found for p27(Kip1). The ICI 182780 effectively abrogated the increased gene expression associated with E(2) for cyclin B1, but not for cyclin A. In contrast, ICI 182780 was ineffective in the case of raloxifene.Raloxifene increased the proliferation of HUVECs in association with enhanced gene expression of cyclins A and B1.