BACKGROUND & AIMS: Reliable community-based colorectal adenoma prevalence estimates are needed to inform colonoscopy quality standards and to estimate patient colorectal cancer risks; however, minimal data exist from populations with large numbers of diverse patients and examiners. METHODS: We evaluated the prevalence of adenomas detected by sex, age, race/ethnicity, and colon location among 20,792 Kaiser Permanente Northern California members >/=50 years of age who received a screening colonoscopy examination (102 gastroenterologists, 2006-2008). RESULTS: Prevalence of detected adenomas increased more rapidly with age in the proximal colon (adjusted odds ratio [OR], 2.39; 95% confidence interval [CI], 2.05-2.80; 70-74 vs 50-54 years) than in the distal colon (OR, 1.89; 95% CI, 1.63-2.19). Prevalence was higher among men vs women at all ages (OR, 1.77; 95% CI, 1.66-1.89), increasing in men from 25% to 39% at >/=70 years and in women from 15% at 50-54 years to 26% (P < .001). Proximal adenoma prevalence was higher among blacks than whites (OR, 1.26; 95% CI, 1.04-1.54), although total prevalence was similar, including persons <60 years old (OR, 1.17; 95% CI, 0.91-1.50). CONCLUSIONS: Prevalence of detected adenomas increases substantially with age and is much higher in men; proximal adenomas are more common among blacks than whites, although the total prevalence and the prevalence for ages <60 years were similar by race. These demographic differences are such that current adenoma detection guidelines may not be valid, without adjustment, for comparing providers serving different populations. The variation in prevalence and location may also have implications for the effectiveness of screening methods in different demographic groups.