Prostate cancer (PC) is the most commonly diagnosed malignancy for men in Western countries. Research showed that cruciferous vegetables containing indole derivatives were involved in cancer prevention. This study was designed to investigate the effect of indole-3-carbinol (I3C) in cell lines and on PC tumor growth in mice when given as a therapeutic and as a preventive treatment. The effect in vitro of 13C on the viability, proliferation and apoptosis of mouse PC cell line TRAMP-C2 and on bovine capillary endothelial (BCE) cells was examined using MTT, BrdU and FACS analyses. The effect of I3C (20 mg/kg body weight) as both a therapeutic and a preventive treatment on the growth of PC cells, inoculated subcutaneously in C57BL/6 mice, was evaluated using tumor volume measurements and immunohistochemistry. I3C decreased the proliferation rate in 3-folds (staining to Ki-67), and promoted apoptosis (staining with caspase 3). I3C, injected intraperitonially (I.P.), significantly inhibited the tumor growth (a 78% decrease in tumor volume) and affected the angiogenesis process by decreasing the microvessel density (CD31 endothelial marker) and complexity. I3C has a significant inhibitory effect on PC cells in vitro and in vivo, and offers a potential usage as both preventive and therapeutic agent for humans.