OBJECTIVE: To identify risk factors for benign prostatic hyperplasia (BPH). SUBJECTS AND METHODS: Medical history data, including reported urological conditions and treatments, and risk factor data were collected from 34 694 participants in the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, a randomized controlled trial designed to evaluate methods for the early detection of cancer. RESULTS: Asian men had the lowest risks (odds ratio, 95% confidence interval) for nocturia (0.7, 0.5-0.9), physician-diagnosed BPH (0.3, 0.2-0.5) and transurethral prostatectomy (TURP, 0.2, 0.1-0.6), while risks for Whites and Blacks were similar for most measures of BPH. Greater alcohol intake was associated with decreased nocturia (P trend = 0.002), BPH (P trend < 0.001) and TURP (P trend < 0.001). Current tobacco use was associated with decreased nocturia (0.8, 0.7-0.9), BPH (0.7, 0.6-0.8) and TURP (0.6, 0.4-0.8) but dose-response patterns were weak. CONCLUSION: Asian-Americans have the lowest risk of clinical BPH. Alcohol and possibly cigarettes are related to a lower risk for BPH.