Background: The pathogenesis of coarctation of aorta (CoA) has not been clearly elucidated. There are hypothesis that CoA patients has abnormal extension of ductal tissue in aorta which has pathogenic role. The phenotypic modulation of smooth muscle cells (SMCs) are known to be involved in some human vascular lesions. The aim of this study was to investigate the abnormally extended ductal tissue in aorta and the phenotypic modulation of smooth muscle cells in CoA by comparative morphologic and immunohistochemical analyses. Materials and Methods: Fifteen cases of surgically resected specimens including coarctation segment (CS), ductus arteriosus (DA) and transition zone (TZ) were reviewed histologically. To determine SMC phenopypes, immunostaining to SM1, SM2, SMemb and 慣-smooth muscle actin were performed. Apoptotic cell death was estimated by TUNEL method. Results: Abnormal extension of ductal tissue was found in the media of TZ and CS in all investigated cases. CS showed similar histologic pattern to closing DA, but intimal migration of SMCs was less prominent in CS. CS showed the least differentiated SMC phenotype among the neighboring vascular regions. Medial SMC marker expression pattern of CS and DA was nearly identical, except for stronger contractile protein expression in DA. Apoptosis in media was frequently found in both coarctation segment and DA. Conclusions: CoA and closing DA undergo same histopathologic processes and apoptosis. CS was composed of the least differentiated SMCs, suggesting that SMC phenotypic modulation is involved in the formation of CoA.