Apolipoprotein E (apoE) is a protein involved in reverse cholesterol transport. Among other tissues, apoE is expressed in macrophages where its expression increases when macrophages develop into foam cells. It has been recently shown that peroxisome-proliferator-activated receptor gamma (PPARγ) is involved in this conversion. Northern-blot analysis was carried out in the macrophage cell line THP1 to determine whether apoE mRNA levels were regulated by ciglitazone, a PPARγ inducer. The results indicated that treatment with ciglitazone doubled the levels of apoE mRNA. To identify a possible PPARγ response element (PPRE), several portions of apoE gene control region were used to construct luciferase reporter plasmids. In U-87 MG cells, a 185bp fragment located in the apoE/apoCI intergenic region was sufficient to induce a 10-fold increase in the luciferase activity of the extract of cells co-transfected with a PPARγ expression plasmid. Subsequent analysis revealed the presence of a sequence with a high level of sequence similarity to the consensus PPRE. Mutations in this sequence resulted in a lack of functionality both in transient transfection and in electrophoretic-mobility-shift assays. These results demonstrated the presence of a functional PPRE in the apoE/apoCI intergenic region. These results have implications for the regulation of apoE gene expression and could be relevant for understanding the anti-atherogenic effect of thiazolidinediones.