The regulation of vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis all play a clear role in the atherosclerotic process. Recently, we reported on the inhibition of the exaggerated growth phenotype of VSMCs isolated from hypertensive rats, by prostaglandin D₂ synthase (L-PGDS). In the present study, we report the differential effects of L-PGDS on VSMC cell cycle progression, migration, and apoptosis in wildtype VSMCs, versus those from a type 2 diabetic model. In wildtype VSMCs, exogenously added L-PGDS delayed serum-induced cell cycle progression from the G1-to-S phase, as determined by gene array analysis and the decreased protein expressions of cyclin dependent kinase-2, p21^Cip1, and cyclin D1. Cyclin D3 protein expression was unaffected by L-PGDS, although its gene expression was stimulated by L-PGDS in wildtype cells. In addition, platelet-derived growth factor-induced VSMC migration was inhibited by L-PGDS in wildtype cells. Type 2 diabetic VSMCs, however, were resistant to the L-PGDS effects on cell cycle progression and migration. L-PGDS did suppress the hyper-proliferation of diabetic cells, albeit through a different mechanism, presumably involving the 2.5-fold increase in apoptosis and the concomitant 10-fold increase of L-PGDS uptake we observed in these cells. We propose that in wildtype VSMCs, L-PGDS retards cell cycle progression and migration, precluding hyperplasia of the tunica media, and that diabetic cells appear resistant to the inhibitory effects of L-PGDS, which consequently may help explain the increased atherosclerosis observed in diabetes.