An isocaloric low-fat diet has been shown to slow androgen-sensitive Los Angeles Prostate Cancer-4 (LAPC-4) tumor growth in a mouse xenograft model. LAPC-4 cells were injected into male severe combined immunodeficient mice. After palpable tumors developed, the mice were divided into three groups, high-fat intact, high-fat castration, and low-fat castration. Tumor latency (18 versus 9 weeks; P < 0.001) and mouse survival (20.8 +/- 1.3 versus 13 +/- 0.7 weeks; P < 0.01) were significantly longer in the low-fat castration versus high-fat castration group. Reduced dietary fat intake delayed conversion from androgen-sensitive to -insensitive prostate cancer and significantly prolonged survival of severe combined immunodeficient mice bearing LAPC-4 xenografts.