Transforming growth factor-β (TGFβ) has a wide range of activities on vascular cells and inflammatory cells, suggesting it may have different functions during various stages of atherogenesis. We report that mice heterozygous for the deletion of the tgfb1 gene (tgfb1^+/− mice) have reduced levels of TGFβ1 in the artery wall until at least 8 weeks of age. On a normal mouse chow diet, the vascular endothelium of tgfb1^+/− mice is indistinguishable from wild-type littermates, assessed by morphology and intercellular cell adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression. In contrast, levels of the smooth muscle isoforms of actin and myosin in medial smooth muscle cells of tgfb1^+/− mice are significantly reduced. Following feeding a cholesterolenriched diet for 12 weeks, high levels of ICAM-1 and VCAM-1 were detected in the vascular endothelial cells of tgfb1^+/− mice, but not wild-type mice. Furthermore, marked deposition of lipid into the artery wall was only observed in the tgfb1^+/− mice on the cholesterol-enriched diet. These vascular lipid lesions were accompanied by local invasion of macrophages. We conclude that deletion of a single allele of the tgfb1 gene results in a reduced level of TGFβ1 antigen in the aorta together with reduced smooth muscle cell differentiation, whereas the addition of a high fat dietary challenge is required to activate the vascular endothelium and to promote the formation of fatty streaks resembling early atherosclerosis in humans.