N-4-Hydroxyphenylretinamide (fenretinide or 4HPR), a derivative of retinoic acid, has been demonstrated to decrease the development of prostate cancer in a rat carcinogenesis model. This study was undertaken to determine if 4HPR is an effective agent for the treatment of established prostate cancer. In vitro, 4HPR was cytotoxic to rat and human prostate cancer cells as well as endothelial cells. Utilizing three different angiogenesis inhibition assays, it was demonstrated that 4HPR inhibited angiogenesis as well as endothelial cell motility and tubule formation. In vivo, 4HPR inhibited prostate cancer growth in a significant manner. These findings suggest that 4HPR may be a potent inhibitor of early prostate cancer growth.