Smooth muscle cell death may contribute to necrotic plaque rupture and subsequent thromboembolus. Stress-induced synthesis of heat-shock proteins (HSPs) normally protects cells from death, but vascular HSPs may become insufficient as cytotoxicity increases in advanced plaques. To determine whether vascular HSP content is altered near necrosis, we compared 70-kD HSP (HSP70) distribution between fibrotic and necrotic plaques in immunostained carotid endarterectomy specimens. Average levels of HSP70 immunoreactivity were compared by video densitometry between fibrotic and necrotic plaques or between their underlying media. Both necrotic plaques and their underlying media contained significantly more HSP70 staining than did fibrotic tissues. To test whether cellular HSP70 correlated with resistance to toxicity in vitro, aortic smooth muscle cells (aSMCs) were heat shocked to induce endogenous HSPs or given 2 to 50 μg/mL purified HSP70. Cells were then serum deprived or exposed to 12 to 96 μmol/L cholestanetriol (C3ol) or 25-hydroxycholesterol, and survival was determined. Cellular HSP70 content was assayed by immunoblotting, and protein synthesis was monitored by ³⁵S radiolabeling. Serum deprivation inhibited general protein synthesis but induced HSP70; C3ol exposure inhibited both overall protein and HSP70 synthesis, including post–heat shock. Induction of endogenous HSPs or 10 μg/mL exogenous HSP70 improved viability of serum-deprived cells (P<.05 and P<.01, respectively), while only exogenous HSP70 protected against C3ol (P<.002). The results suggest that insufficient HSP70 accumulates in aSMCs residing near necrosis to protect against plaque toxicity; aSMC death might then occur, allowing resident macrophages to degrade and destabilize the matrix, leading to rupture.