Background and objective: Observational studies have reported that estrogen-progestin therapy as well as estrogen therapy might prevent cardiovascular disease (CVD) in postmenopausal women. Clinical trials using conjugated equine estrogen and medroxyprogesterone acetate (MPA) demonstrated no effect overall in both secondary and primary prevention of CVD. Progestin might oppose beneficial effects of estrogen. Our group published estradiol (E₂) inhibits proliferation of vascular smooth muscle cells (VSMC) induced by lysophosphatidylcholine (lysoPC), an active component of oxidized LDL. The purpose of this study is to examine the effects of progestogen on VSMC proliferation. Materials and methods: VSMC were derived from female rat aorta. To study cell proliferation, DNA systhesis was measured by [³H] thymidine incorporation. Intracellular formation of reactive oxygen species (ROS) was visualized by using oxidation-sensitive probe, 2,7-dichlorofluorescin diacetate under laser scanner confocal microscopy and quantified by flow cytometric assay. Results: Progesterone (P₄) blocked [³H] thymidine incorporation in a dose-dependent manner. In addition, P₄ reduced ROS production stimulated with lysoPC. Promegestone (R5020), a specific progesterone receptor (PR) agonist, also decreased cell proliferation. P₄ (10^-8 M) in combination with E₂ (10^-8 M) further diminished cell proliferation. MPA showed similar inhibitory action and did not attenuate estrogen effect. Conclusions: Progestogens inhibits VSMC proliferation induced by lysoPC via an antioxidant and PR-mediated mechanism. Progestogens do not offset antimitogenic effect of estrogen.