The aim of this study was to evaluate the effects of 9-cis retinoid acid (9-cis RA) and all-trans RA (ATRA) on proliferation, migratory ability, synthesis of extracellular matrix, intracellular signal transduction, and differentiation of human aortic smooth muscle cells (haSMCs) in vitro. Changes of cell proliferation following incubation with RAs in different doses (10-6 M, 10-7 M, and 10-8 M) were determined directly by proliferation kinetics and indirectly by bromodeoxyuridine enzyme-linked immuno sorbant assays and colony-formation assays. The migratory ability of haSMCs was examined with the help of migration assays. The production of the extracellular matrix protein tenascin was explored by immunostaining. The amounts of total p44/p42 mitogen-activated protein kinases (MAPKs) and their phosphorylated forms were detected with the help of Western blots. To judge the state of differentiation of haSMCs, cell cycle distribution and the pattern of α-actin were analyzed. Both RAs clearly inhibited the proliferation of haSMCs in a dose-dependent manner. 9-cis RA had a tendency to be more effective than ATRA. After treatment with RAs, the migratory ability was especially reduced during stimulation with platelet-derived growth factor (PDGF) and the synthesis of tenascin decreased. Although the total p44/p42 MAPKs were downregulated, the amounts of activated forms increased markedly in the cells incubated with RAs and particularly stimulated with PDGF. The cell-cycle analysis demonstrated an increased G 1-phase, complemented by a stronger expression of α-actin after treatment. 9-cis RA especially has the potential to inhibit the proliferation, migration, and synthesis of extracellular matrix of haSMCs by inducing differentiation in vitro.