BACKGROUND: Despite growing interest in prevention of lower urinary tract symptoms (LUTS) through better understanding of modifiable risk factors, large-scale population-based evidence is limited. OBJECTIVE: To describe risk factors associated with severe LUTS in the 45 and Up Study, a large cohort study. DESIGN, SETTING, AND PARTICIPANTS: A cross-sectional analysis of questionnaire data from 106,435 men aged ≥ 45 years, living in New South Wales, Australia. OUTCOME MEASURES AND STATISTICAL ANALYSIS: LUTS were measured by a modified version of the International Prostate Symptom Score (m-IPSS). The strength of association between severe LUTS and socio-demographic, lifestyle and health-related factors was estimated, using logistic regression to calculate odds ratios, adjusted for a range of confounding factors. RESULTS: Overall, 18.3% reported moderate, and 3.6% severe, LUTS. Severe LUTS were more common among men reporting previous prostate cancer (7.6%), total prostatectomy (4.9%) or having part of the prostate removed (8.2%). After excluding men with prostate cancer or prostate surgery, the prevalence of moderate-severe LUTS in the cohort (n = 95,089) ranged from 10.6% to 35.4% for ages 45-49 to ≥ 80; the age-related increase was steeper for storage than voiding symptoms. The adjusted odds of severe LUTS decreased with increasing education (tertiary qualification versus no school certificate, odds ratio (OR = 0.78 (0.68-0.89))) and increasing physical activity (high versus low, OR = 0.83 (0.76-0.91)). Odds were elevated among current smokers versus never-smokers (OR = 1.64 (1.43-1.88)), obese versus healthy-weight men (OR = 1.27 (1.14-1.41)) and for comorbid conditions (e.g., heart disease versus no heart disease, OR = 1.36 (1.24-1.49)), and particularly for severe versus no physical functional limitation (OR = 5.17 (4.51-5.93)). CONCLUSIONS: LUTS was associated with a number of factors, including modifiable risk factors, suggesting potential targets for prevention.