Retinoids inhibit rat vascular smooth muscle cell (VSMC) proliferation in vitro and intimal hyperplasia in vivo. We examined the mechanism of the antiproliferative effect of retinoids on human coronary artery smooth muscle cells (human CASMCs). The RAR ligands all-trans-retinoic acid (atRA) and ethyl-p-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-l-propenyl]-benzoic acid (TTNPB); a pan-RXR/RAR agonist, 9-cis-retinoic acid (9cRA); and the RXR-selective ligand AGN4204 all inhibited DNA synthesis stimulated with platelet-derived growth factor and insulin (IC(50): TTNPB 63 nmol/L, atRA 120 nmol/L, AGN4204 460 nmol/L, 9cRA 1.5 micromol/L). All retinoids blocked cell cycle progression as determined by flow cytometry and inhibited retinoblastoma protein (Rb) phosphorylation. TTNPB, atRA, and AGN4204 inhibited the mitogenic induction of cyclin D1, whereas 9cRA had no effect. None of the retinoids affected the expression of CDK 2, 4, or 6 or cyclin E. All retinoids attenuated mitogen-induced downregulation of CDKI p27(Kip1), a major negative regulator of Rb phosphorylation, partly through stabilizing p27(Kip1) turnover. These data demonstrate that retinoids have antiproliferative activity by modulating G(1) --> S cell cycle regulators in human CASMCs through inhibition of Rb phosphorylation and elevation of p27(Kip1) levels.