While the role of dietary fats in breast cancer remains controversial, the recent cloning of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor, from human breast cancer cells lines provides a potential molecular link. Several fatty acids from four classes of dietary fats were tested for their ability to mediate the transcriptional activity of PPARgamma in MCF-7 and MDA-MB-231 cells using growth media with minimal serum. Whereas omega-3 fatty acids inhibit transactivation of PPARgamma to levels below control, omega-6, monounsaturated and saturated fatty acids stimulate the activity of the transcriptional reporter. These studies indicate that individual fatty acids differentially regulate the transcriptional activity of PPARgamma by selectively acting as agonists or antagonists. Furthermore, the transcriptional activation of PPARgamma correlates with cell proliferation in MCF-7 cells. Understanding the effects of individual fats on breast cancer cells and PPARgamma transactivation could provide important new insights into the epidemiology of breast cancer and the role of dietary fat.