Technical Abstract: All-trans retinoic acid (ATRA) promotes terminal differentiation in epithelial cells and anti-angiogenesis and thus, may have beneficial effects in an intervention therapy for prostate cancer. We used the autochthonous spontaneous transgenic adenocarcinoma of the mouse prostate (TRAMP) model system to test the ability of ATRA to prevent initiation and progression of prostate cancer in a pre-clinical setting. Initial studies demonstrated that exposure of TRAMP-derived C2N prostate tumor cells to ATRA in vitro decreased total viable cell numbers with a concomitant decrease in the fraction of cells in S phase. When TRAMP mice were treated in vivo with ATRA for either 6 or 8 weeks at low, medium, or high dose, mice on average presented with lower grade and more differentiated tumors. However, ATRA therapy conferred no significant protection on incidence of tumors or frequency of metastasis at any dose. Nevertheless, we were able to observe a significant decrease in the expression of synaptophysin, a marker of neuroendocrine differentiation, in tumors of mice receiving the highest dose of ATRA. As well, expression of the cell cycle inhibitor p21 was found to be elevated only in well-differentiated tumors of mice, treated with ATRA while expression of p27, was found to be elevated only in the poorly differentiated tumors. Collectively, our in vitro and in vivo data demonstrates that ATRA was able to slow prostate tumor cell proliferation, induce apoptosis, and block the emergence of the neuroendocrine phenotype. Furthermore, our study suggests the differential regulation of p21 and p27 as a molecular mechanism whereby ATRA intervention therapy can inhibit the natural history of spontaneous prostate cancer.