The peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-dependent transcription factor that has been demonstrated to regulate fat cell development and glucose homeostasis. PPARγ is also expressed in a subset of macrophages and negatively regulates the expression of several proinflammatory genes in response to natural and synthetic ligands. We here demonstrate that PPARγ is expressed in macrophage foam cells of human atherosclerotic lesions, in a pattern that is highly correlated with that of oxidation-specific epitopes. Oxidized low density lipoprotein (oxLDL) and macrophage colony-stimulating factor, which are known to be present in atherosclerotic lesions, stimulated PPARγ expression in primary macrophages and monocytic cell lines. PPARγ mRNA expression was also induced in primary macrophages and THP-1 monocytic leukemia cells by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). Inhibition of protein kinase C blocked the induction of PPARγ expression by TPA, but not by oxLDL, suggesting that more than one signaling pathway regulates PPARγ expression in macrophages. TPA induced the expression of PPARγ in RAW 264.7 macrophages by increasing transcription from the PPARγ1 and PPARγ3 promoters. In concert, these observations provide insights into the regulation of PPARγ expression in activated macrophages and raise the possibility that PPARγ ligands may influence the progression of atherosclerosis.