Phenotypic change of vascular smooth muscle cells (VSMCs) from differentiated to dedifferentiated state accompanies the early stage of atherosclerosis and restenosis. Although much progress has been made in determining the molecular mechanisms involved in VSMC dedifferentiation, research on VSMCs redifferentiation is hindered by the lack of an appropriate complete redifferentiation model. We established an in vitro model of redifferentiation by using postconfluent VSMCs from human umbilical artery. We demonstrated that serum-deprived VSMCs are capable of complete redifferentiation. After serum deprivation, the postconfluent cultured human umbilical VSMCs became elongated and spindle-shaped, with elevation of myofilament density, and reacquired contraction. Expressions of VSMC-specific contractile proteins, such as SM -actin, SM-MHC, calponin, and SM 22, were increased and reached to the levels in differentiated cells after serum deprivation. To determine the molecular mechanism of the phenotypic reversion, the levels of expression, phosphorylation, and binding activity of serum response factor (SRF), a key phenotypic modulator for VSMCs, were measured. The results showed that SRF binding activity with CArG motif was significantly increased after serum deprivation, whereas no changes were found in SRF expression and phosphorylation. The increased SRF binding activity is accompanied by an increase in expression of its coactivators such as myocardin. Furthermore, the phenotypic reversion was markedly inhibited by decoy double-strand oligodeoxynucleotides (ODNs) containing SM -actin CArG motif, which was able to competitively bind to SRF. The results suggested that serum deprivation results in redifferentiation of human umbilical VSMCs. This novel model of VSMC phenotypic reversion should be valuable for research on vascular disease.