4-Hydroxynonenal (4-HNE) is a peroxidation product of ˆ-6-poly-unsaturated fatty acids and exerts growth modifying as well as cytotoxic activities. This aldehyde component of oxidized lipid is increased during the aging process. In this study, to characterize the potential role of the lipid peroxidation product in aging, we studied the effects of 4-HNE on cell proliferation and activation of cell-cycle machinery and the mitogen-activated protein kinase signaling pathway. 4-HNE-treated smooth muscle cells (SMCs) have shown a different cell proliferation rate depending on 4-HNE's incubation time and concentration. Interestingly, a prolonged treatment of 0.1 μM 4-HNE (36 h) resulted in an increase of cell growth in young SMCs but displayed cytotoxicity in aged SMCs. Treatment with 4-HNE enhanced cyclin D1 expression and activation of the extracellular signal-regulated kinase (ERK) signaling pathway, which were stronger in young SMCs compared with aged SMCs. Moreover, 4-HNEinduced cell proliferation and cyclin D1 expression were significantly attenuated by PD98059, the ERK inhibitor, in young SMCs. These data clearly indicate that increased cell proliferation was associated with the induction of cyclin D1 expression which was regulated by ERK in 4-HNE-treated young SMCs for 36 h. In contrast, we found that the cytotoxicity of aged SMCs to 4-HNE was partly related to generation of ROS and that pretreatment with N-acetyl-Lcysteine prevented 4-HNE-induced cell death in aged SMCs. These results suggest that the prolonged treatment of 0.1 μM 4-HNE-induced cell growth inhibition was caused by generation of ROS. Collectively, the age-related different growth rates and responses to 4-HNE are related to the expression level of cyclin D1, activation of the ERK signaling pathway, and regulation of ROS generation in SMCs.