2500 Breast cancer remains the second leading cause of cancer-related deaths in American women, and to date, no effective treatment for metastatic disease exists. Clinical and pre-clinical studies have linked n-6 polyunsaturated fatty acids with mammary carcinogenesis. We investigated the cellular and molecular effects of linoleic acid on the human breast cancer cell line T47D. The XTT cell viability assay indicated that linoleic acid induced a significant and dose dependent (0-10,000 ng/ml) response in cell viability compared to controls. Maximal cell viability (56% over control values) was observed at 100 ng/ml. A 96 hour cell proliferation assay with 100 ng/ml of linoleic acid resulted in an approximately 35% increase in T47D cell number, compared to controls. Flow cytometric analysis indicated that the increase in cell number was accompanied by an approximately 50% increase in the number of cells in the S phase of the cell cycle (from 17.1% to 28.5% at 12 hours and from 14.1% to 21.0% at 24 hours). Affymetrix Human Genome U95Av2 Array GeneChips microarray analyses indicated that of the 12,558 genes on the microarray, approximately 5,795 genes were expressed in treated and untreated T47D cells, with 35 genes up-regulated and 9 genes down-regulated in linoleic acid treated cells. MAPPFinder analysis indicated that the highest Z scores for GO terms were associated with the biological “process” of transcription regulation, the nuclear component, and the DNA binding, protein serine/threonine kinase, and transcription factor molecular functions. MAPPs for biologically functional groups and pathways indicated that linoleic acid affected changes in transcriptional regulation, cell cycle, G protein signaling and the MAP kinase signal transduction cascade. Real-time RT-PCR was used to confirm microarray data in a select set of genes within these pathways.Supported by NCI grant 1R21CA88982-0-1 and AICR grant #97A072. Niradiz Reyes is the recipient of a Fulbright/Laspau/Colciencias scholarship.