BACKGROUND: Although benign prostate hyperplasia (BPH) and prostate cancer (PCa) share features such as hormone-dependent growth and response to treatment with antiandrogen therapy, BPH is generally not considered a premalignant lesion. OBJECTIVE: To determine whether clinical BPH is associated with an increased risk of PCa incidence and mortality. DESIGN, SETTING, AND PARTICIPANTS: Using designs with individual participant data from five national registries, we studied the entire Danish male population from 1980 through 2006, a total of 3 009 258 Danish men. We collected PCa diagnoses (n=53 315), information on PCa mortality (n=25 459), and ascertained clinical BPH (not histologically proven BPH) through hospitalization (n=187 591) and/or surgery (n=77 698) from 1980 to 2006 and the use of a-adrenergic receptor antagonists (n=143 365) and/or the use of 5a-reductase inhibitors (5-ARIs) (n=47 465) from 1995 to 2006. MEASUREMENTS: PCa incidence and mortality was assessed for each category of clinical BPH using Kaplan-Meier plots of cumulative incidence and Cox proportional hazard ratios (HRs) adjusted for potential confounders. RESULTS AND LIMITATIONS: For the entire cohort studies, multivariate-adjusted HRs for PCa incidence were 2.22 (95% confidence interval, 2.13-2.31) in men hospitalized and 3.26 (3.03-3.50) in men operated on for clinical BPH versus general population controls. Corresponding HRs for PCa mortality were 2.00 (1.91-2.08) for hospitalization and 7.85 (7.40-8.32) for surgery. For age-matched cohort studies, corresponding HRs for PCa incidence were 3.04 (2.96-3.13) for hospitalization, 2.60 (2.47-2.73) for surgery, 4.49 (4.33-4.65) for a-adrenergic receptor antagonist use, and 2.54 (2.40-2.68) for 5-ARI use. Each category of clinical BPH has limitations, but limitations differ between the categories and therefore are unlikely to explain the results. CONCLUSIONS: In Danish men followed for up to 27 yr, clinical BPH was associated with a two- to three-fold increased risk of PCa incidence and with a two- to eight-fold increased risk of PCa mortality. These data should not be used to infer causality.