Beginning January 1, 2006 the U.S. Food and Drug Administration mandated that the amount of trans fats per serving be listed on the Nutrition Facts panel. Consequently new soybean breeds that would no longer be subject to the hydrogenation process, thus reducing trans fats, were developed. By traditional plant breeding techniques, plant breeders have developed a low linolenic soybean with 83.36% less alpha-linolenic acid (ALA; omega-3) than conventional soybean. A number of studies have demonstrated that the influence of dietary fats on cancer depends on the quantity as well as the type of lipids and diets with a disproportionately high omega-6 (n-6)/omega-3 (n-3) ratio are thought to contribute to cardiovascular disease, inflammation and cancer. Conventional soybean oil (SO) has an n-6/n-3 ratio of 8/1 while the new low linolenic soy bean oil (LLSO) has an n-6/n-3 ratio of 56/1. In this study, we evaluated the effects of dietary LLSO, SO and lard on the progression of breast cancer (BC). Thirty-five, 6-wk old, ovariectomized, athymic mice received human pre-malignant breast cells (MCF-10AT1 1 x 105 cells/40μl/ Matrigel/spot, 4 spots/mouse). Mice were divided into three groups and then fed isocaloric and isonitrogenous diets with disparate fat sources: LLSO (20% of total energy intake), SO (20%) and lard (20%). The dietary treatment lasted 24 weeks upon which the study was terminated and tumors, tissues and blood samples were analyzed. Average tumor surface area at termination for the LLSO group was 45.11 ± 4.46 mm2, 40.08 ± 4.2 mm2 for lard and 56.63 ± 5.42 mm2 for SO. Messenger RNA (mRNA) expression of HER2/neu, epidermal growth factor receptor (EGFR), H-ras, Bcl-2, cyclooxygenase-2 (COX-2), vascular epidermal growth factor (VEGF), and fatty acid synthase (FAS) in tumors were analyzed using quantitative real time-polymerase chain reaction (qRT-PCR). We found that dietary LLSO supplementation significantly (p < 0.05, Tukey's test) increased tumor expression of oncogenes HER2/neu, EGFR, FAS, and H-ras, but not in the SO or lard supplemented groups. Relative mRNA expression was also significantly increased in both LLSO and SO groups, however, there was no marked difference in mRNA expression for Bcl-2 and COX-2. Removed tumors were evaluated microscopically for histologic lesion progression corresponding to human breast cancer progression. Tumors from the LLSO group showed more advanced lesions (grade 2) (p < 0.05, Chi Square test) with areas of four or more layers of epithelial cells and irregularly shaped lumens. These data suggest that dietary intake of LLSO may accelerate mammary tumor progression at a faster rate than conventional SO or lard. / Master of Science