We studied the effect of adenovirus-mediated gene transfer of p53 protein on the injured rat carotid artery to determine its ability to decrease the formation of intimal hyperplasia.In vivo gene transfer was used in isolated segments of balloon injured rat carotid arteries. Genetically modified adenovirus encoding for wild-type p53 protein (AdWTp53) was applied in three different concentrations. Control rats received either adenovirus null (AdNull), 8 × 10 10 pfu/ml or M-199 medium solution (vehicle). Expression of p53 was determined 4 days after gene transfer by Western-blot. Intimal hyperplasia was assessed after 14 days by harvesting carotid arteries and determining the intima/media (I/M) ratio by cross-sectional area measurement. Simultaneously, immunohistochemical analysis was done to detect the presence of p53 on smooth muscle cell nuclei.p53 expression was confirmed by Western-blot. There was a significant reduction in intimal hyperplasia on all treated animals as compared with controls. The highest dose of AdWTp53 resulted in a near total arrest of intimal hyperplasia (mean of 97% reduction). A dose-response curve was observed. The immunohistochemical analysis was positive for the presence of p53 in rats infected with AdWTp53. The AdWTp53 groups have three different concentrations: #1, 8 × 10 10 pfu/ml; #2, 1.6 × 10 10 pfu/ml; and #3, 8 × 10 9 pfu/ml.Adenovirus-mediated gene transfer of p53 protein significantly decreases the formation of intimal hyperplasia in the rat carotid injury model in a dose-response manner. This may represent a potential therapy for restenosis in humans.