T-cell death associated gene 51 (TDAG51) is a pro-apoptotic gene that can be induced by endoplasmic reticulum (ER) stress agents, including homocysteine, tunicamycin, thapsigargin or dithiothreitol. Our previous studies have demonstrated that transient overexpression of TDAG51 elicited significant changes in cell morphology, decreased cell adhesion and promoted detachment-induced programmed cell death (PCD). In support of these in vitro findings, we have further shown that TDAG51 expression was increased and correlated with PCD in the atherosclerotic lesions from apolipoprotein E (apoE)-deficient mice fed hyperhomocysteinemic diets, compared to mice fed control diet. We designed the current study to investigate the effect of TDAG51 deficiency in the development and progression of atherosclerosis. To assess in vivo significance of TDAG51 on atherosclerosis, we have crossed TDAG51-deficient mice with apoE-deficient mice to obtain double knockout mice. Our findings have demonstrated that TDAG51/apoE-deficient mice have a significant decrease in atherosclerotic lesion area, compared to age- and sex-matched apoE-deficient mice. Total plasma cholesterol and triglycerides as well as lipoprotein profiles were similar in both groups. However, TDAG51/apoE-deficient mice presented with increased hepatic steatosis. Further, a significant upregulation of peroxisome proliferator-activated receptor γ (PPAR-γ), a transcription factor required for adipose tissue formation, was demonstrated in TDAG51-deficient mouse embryonic fibroblasts (MEFs), compared to control wildtype MEFs. Interestingly, earlier studies in mice have reported that overexpression of PPAR-γ decreases atherosclerotic lesion development and increases hepatic steatosis - a phenotype similar to that observed in the mouse deficient in both apoE and TDAG51. Collectively, these findings provide evidence that TDAG51 mediates atherosclerotic lesion development and hepatic steatosis through a mechanism involving PPAR-γ.