Fat Specific Protein 27 (FSP27)/Cidec was initially identified by its upregulation in TA1 adipogenesis and is one of three CIDE family proapoptotic proteins. Ectopic expression of CIDEs causes apoptosis of mammalian cells. On the other hand, FSP27 has recently been illustrated to regulate lipid droplet size and promote lipid storage in adipocytes. Regulation of endogenous FSP27 expression is unknown. We assessed FSP27 transcript level in the well-characterized 3T3-L1 in vitro adipocyte differentiation model and found its emergence parallels the adipocyte-enriched transcripts aFABP and SCD1. Furthermore, FSP27 is a differentiation-dependent transcript in adipogenesis of primary rodent and human preadipocytes and in brown adipogenesis. FSP27 transcript is inversely regulated by TNF and insulin, consistent with an antilipolytic function. It is nearly abolished with a 4 h exposure of 3T3-L1 adipocytes to 10 ng/ml TNF, while treatment with 100 nM insulin increased FSP27 transcript 8-fold; LY294002 blocked this response indicating involvement of PI3-kinase signals. Northern blot analysis of murine tissues indicated exclusive expression of FSP27 in white and brown adipose tissue; however a dramatic upregulation occurred in liver of ob/ob mice. Ectopic expression of murine FSP27 in 293T cells and in 3T3-L1 preadipocytes led to the appearance of key apoptotic hallmarks and cell death. However, despite the upregulation for FSP27 in adipogenesis we failed to detect DNA laddering indicative of apoptosis in 3T3-L1 adipocytes. This suggests that adipogenesis is accompanied by decreased susceptibility to the proapoptotic effects of FSP27. Overall, our findings support roles for FSP27 in cell death and in adipocyte function