Previously we have shown that the chromatin-modifying high mobility group A1 protein (HMGA1) is required for Peroxisome Proliferator-activated Receptor gamma (PPARgamma)-mediated transrepression in vascular smooth muscle cells (VSMCs). The aim of the present study was to characterize the in-vivo relevance of HMGA1 in PPARgamma-mediated beneficial effects after vascular injury. The effect of pioglitazone (pio) treatment in wildtype (wt) and HMGA1-deficient mice on neointima formation was investigated in a mouse femoral artery after wire-induced injury. Arterial injury resulted in a prominent formation of a neointima in wt and HMGA1-deficient mice (neointima/media (I/M) ratio vs. uninjured contra-lateral artery 1.59±0.13 (wt-mice) and 1.69±0.13 (HMGA1-deficient mice)). In wt mice, 3 weeks peri-/post-injury treatment with pio (10mg/kg/d) led to significant 50% reduction of neointima formation (p<0.01). In contrast, the protective actions of pio were completely abolished in HMGA1-deficient mice. In the mouse vascular injury model, the matrix-degrading matrix metalloproteinase-9 (MMP-9) mediates new forming neointima in injured vessels, and thereby substantially drives vascular pathology. It has been reported that inhibition of MMP-9 is potently blocked by glitazone-activated PPARgamma. Thus, we examine the regulation of MMP-9 gene expression in the femoral artery of wt and HMGA1-deficient mice after vascular injury in the presence of pio-mediated PPARgamma activation. In wt mice injury-mediated vascular MMP-9 upregulation was reduced in the presence of pio-mediated PPARgamma activation (100%±38.87 vs. 36.10%±18.39, p<0.05). According to the lack of PPARgamma-mediated repression of MMP-9 in-vitro in the absence of HMGA1, the reduction of vascular MMP-9 expression by pio was completely abolished in HMGA1-deficient mice (100%±38.87 vs. 114.51%±35.20, p=n.s.). To summarize these data confirm the importance of the PPARgamma-HMGA1 pathway for the vascular protective actions of PPARgamma-activating glitazones involving vascular MMP-9 regulation. The results identify HMGA1 as a substantial new regulator for PPARgamma-mediated inhibition of neointima formation. These data supply new information on the PPARgamma-dependent vascular transcriptional network, and support the understanding of molecular consequences of glitazone-therapy in the vasculature.