Objective-Intimal smooth muscle cells (SMCs) are dedifferentiated SMCs that have a powerful ability to proliferate and migrate. This cell-type is responsible for the development of intimal hyperplasia after vascular angioplasty. Retinoids, especially all-trans retinoid acid, are known to regulate many processes activated at sites of vascular injury, including modulation of SMC phenotype and inhibition of SMC proliferation. Intracellular levels of active retinoids are under firm control. A key enzyme is the all-trans retinoic acid-degrading enzyme cytochrome p450 isoform 26 (CYP26 10 Retinoids inhibit intimal hyperplasia after vascular injury 6,9 by reducing SMC proliferation. 11 They are known to influence differentiation of various cell types, and previous studies indicate that retinoids can alter the phenotype of vascular SMCs. 9,12 To date, a number of genes have been identified as markers of intimal SMCs including tropomyosin 4, 13 cytokeratins, 14 and cellular retinol binding protein-1 (CRBP-1). 15 Interestingly, CRBP-1 is involved in retinoid metabolism, 16 suggesting an altered retinoid metabolism in the intima. Indeed, we previously demonstrated that intimal SMCs have higher capacity to metabolize retinol into the active metabolite, all-trans retinoic acid (atRA), than medial SMCs. 17 The inhibition of DNA synthesis in intimal SMCs, in contrast to increased DNA synthesis in medial SMCs on retinol stimulation, 17 further supports the concept of differential retinoid metabolism in these subsets of cells.The metabolism of retinoids is complex and involves multiple binding proteins and metabolizing enzymes. The synthesis of retinoic acid (RA) is a 2-step reaction involving substrate-specific enzymes and cellular retinol binding proteins. The first step is the reversible oxidation of retinol (ROH) into retinal by retinol dehydrogenases (RDH). 16,18 The second step, the oxidation of retinal into the biologically active RA, is carried out by retinal dehydrogenases (RalDH). RA activates 2 families of nuclear receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are ligand-inducible transcription factors. Retinoids exert their effects on gene transcription mainly through 2 active stereoisomers, atRA and 9-cis retinoic acid (9-cisRA). 19