Abstract: The invention relates to a method to predict a patient's risk of developing prostate cancer, or to distinguish indolent vs. aggressive cancers among patients suspected of bearing PCa and therefore selected for biopsy, and/or predict the risk of progression of prostate cancer in a patient having undergone prostate cancer treatment, particularly by radical prostatectomy, brachytherapy, and/or external beam radiation therapy, wherein said method comprises detecting/quantifying the level of indoleamine-2,3-dioxygenase (IDO) mRNA or protein in a urine sample obtained from said patient.