The optimal strategy for evaluating rectal bleeding in young persons is unknown. This study examines the prevalence of adenomatous neoplasms identified at endoscopy for rectal bleeding. Retrospective cross-sectional analysis was made of patients under 50 years of age undergoing elective outpatient colonoscopy or flexible sigmoidoscopy for hematochezia. In all, 570 patients (309 F/261 M) met our inclusion criteria. The prevalence of neoplasms was 3.8% (95% CI 1.2–8.5%) among persons under 30, 2.8% (95% CI 0.9–6.3%) among persons age 30–39, and 10.9% (95% CI 7.4–15.4%) among persons age 40–49. The prevalence of neoplasms was higher in persons over age 40 (relative risk 3.43, 95% CI 1.70–6.94). Six of seven advanced neoplasms were identified in persons over age 40 (relative risk 7.4, 95% CI 0.89–60.7). In conclusion, the prevalence of colonic neoplasms in patients 40–50 years old with hematochezia is substantial. Among those persons younger than 40 years, the prevalence of colonic neoplasms is significantly lower.