Objectives. To investigate the effects of flaxseed supplementation on prostatic neoplasia in the transgenic adenocarcinoma mouse prostate (TRAMP) model. Methods. A total of 135 male TRAMP mice 5 to 6 weeks old were randomized to a control group (AIN-76A diet) or an experimental group (AIN-76A diet plus 5% flaxseed by weight). One half of the mice in each group were treated for 20 weeks and the remainder for 30 weeks. At autopsy, urogenital tissues (four prostatic lobes, seminal vesicles, and emptied bladder), lungs, lymph nodes, and grossly abnormal tissues were collected for histologic evaluation. Results. Of the control mice, 100% developed prostate cancer versus 97% of the mice in the flaxseed group. The tumor/urogenital weight was 3.6 ± 0.4 g in the controls versus 1.9 ± 0.2 g in the flaxseed-treated mice (P = 0.0005). At 20 weeks, no significant difference in tumor grade was seen between the two groups; however, at 30 weeks, the flaxseed-treated mice had significantly less aggressive tumors than did the controls (P = 0.01). The prevalence of lung and lymph node metastases was 13% and 16%, respectively, in the control mice versus 5% and 12%, respectively, in the experimental group (difference not significant). After 20 weeks of treatment, cellular proliferation (Ki-67) differed significantly between the control and experimental groups (38.1 ± 2.03 versus 26.2 ± 2.03; P <0.0001), and the apoptotic index (deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling) was 1.45 ± 0.14 versus 3.3 ± 0.31 (P <0.0001). Similar differences were seen after 30 weeks of treatment. Conclusions. A diet supplemented with 5% flaxseed inhibits the growth and development of prostate cancer in the TRAMP model. © 2002, Elsevier Science Inc.