OBJECTIVES Propofol is a widely used intravenous anesthetic agent with antioxidant properties. However, the effect of propofol on reactive oxygen species-induced injury in vascular smooth muscle cells is still unknown. In this study, the authors determined the effect of propofol on hydrogen peroxide-induced injury in vascular smooth muscle cells and the potential molecular mechanisms involved. DESIGN Prospective cell and animal study. SETTING University research laboratory. SUBJECTS Sprague-Dawley rats. INTERVENTIONS For the in vitro study, rat vascular smooth muscle cells pretreated with vehicle or hydrogen peroxide (200 μM) were exposed to vehicle or increasing concentrations of propofol (10-50 μM). For the in vivo study, propofol (12 mg kg⁻¹/hr⁻¹, intravenous) or vehicle was administrated into rats after carotid artery angioplasty. MEASUREMENTS AND MAIN RESULTS The cell survival and cell death were measured by MTT and trypan blue exclusion. Cell apoptosis was evaluated by terminal deoxynucleotide transferase dUTP nick end labeling staining and cleaved caspase-3 expression. To further elucidate the molecular mechanisms in propofol-mediated cellular effect, the expression of programmed cell death 4 and microRNA-21 were measured. Unexpectedly, propofol exacerbated hydrogen peroxide-induced injury responses in vascular smooth muscle cells as demonstrated by a decrease in cell viability and an increase in trypan blue-stained cells, cell apoptosis, and cleaved caspase-3 expression. In addition, propofol inhibited hydrogen peroxide-induced up-regulation of microRNA-21 and increased its target gene programmed cell death 4. Propofol-mediated injury was attenuated by restoration of microRNA-21 expression. Finally, the pro-injury effect of propofol on vascular cells with increased reactive oxygen species was illustrated in vivo in rat carotid arteries after angioplasty. CONCLUSIONS The results revealed that propofol exacerbates cell injury in vascular smooth muscle cells with increased reactive oxygen species, at least in part, through microRNA-21 and its target gene, programmed cell death 4. Because increased reactive oxygen species is a common pathologic component in many vascular diseases, the novel findings in the current study suggest that propofol might have some application limitations.