ARI109924 will be a 2-year, multicentre, randomised, double-blind, placebo-controlled trial assessing the efficacy and safety of dutasteride in extending time to prostate specific antigen (PSA) doubling in men who have been treated for clinically localised prostate cancer (PCa) with a radical therapy (radical prostatectomy, primary radiotherapy or salvage radiotherapy) with curative intent but who experience a biochemical failure (PSA rise) afterwards without signs or symptoms of metastases.