Background: Rising prostate-specific antigen (PSA) levels after radical therapy are indicative of recurrent or residual prostate cancer (PCa). This biochemical recurrence typically predates clinically detectable metastatic disease by several years. Management of patients with biochemical recurrence is controversial. Objective: To assess the effect of dutasteride on progression of PCa in patients with biochemical failure after radical therapy. Design, setting, and participants: Randomised, double-blind, placebo-controlled trial in 294 men from 64 centres across 9 European countries. Intervention: The 5α-reductase inhibitor, dutasteride. Outcome measurements and statistical analysis: The primary end point was time to PSA doubling from start of randomised treatment, analysed by log-rank test stratified by previous therapy and investigative-site cluster. Secondary end points included time to disease progression and the proportion of subjects with disease progression. Results and limitations: Of the 294 subjects randomised (147 in each treatment group), 187 (64%) completed 24 mo of treatment and 107 discontinued treatment prematurely (71 [48%] of the placebo group, 36 [24%] of the dutasteride group). Dutasteride significantly delayed the time to PSA doubling compared with placebo after 24 mo of treatment (p