Technical Abstract: Dietary fat is linked to prostate cancer (PCa), the most commonly diagnosed male cancer, but the nature and strength of the relationships between total fat, n-6 and n-3 fatty acids and PCa remain incompletely understood. Transgenic adenocarcinoma of the mouse prostate (TRAMP) mice (N=10-12 per group) at 8 weeks old were fed ad lib three semisynthetic diets with similar fatty acid speciation: 2 moderately high fat diets (28% as energy (EN)) containing either whole walnuts (WW) or soybean oil (HF) or a lowfat (LF) 12% as EN from soybean oil. Prostates were harvested after 9, 18 and 24 weeks. At 18 weeks, WW animal whole prostate weight (1.28±0.1 g) was lower versus HF mice (2.59 ± 0.7 g) and had reduced plasma IGF-1 (WW 58699±6556 pg/dL vs. 84363±7571) HF, PAI-1 (1.7±0.6 vs. 3.8 ±0.6 (HF)) and LDL cholesterol (14.1 ±5.8 mg/dL) versus HF (25.5 ±12.7 mg/dL); all p<0.05 compared to HF). Prostate growth rate declined (-3± 2%/wk, p<0.015) in both WW and LF groups compared to HF growth rate (11 ± 3 %/week) while WW and HF whole animal growth was equivalent but reduced in LF (p<0.001). Liver metabolomic analysis found WW altered hepatic glucose, oxidative stress as well as glycerolipid, sphingolipid and cholesterol metabolism. The results suggest that walnuts may act via a combination of effects on growth related systems such as IGF-1, sphingolipid and cholesterol. Total dietary fat content did not predict prostate tumor response strengthening the case that it is unlikely that PCa risk is linked to a single nutrient or set of nutrients.