Study Objective: To determine the sensitivity, specificity, and predictive value of Hemoccult II tests for detecting colorectal neoplasm (colorectal carcinoma or polyp or both). Study Design: Prospective analyses of asymptomatic patients ( ≥ 45 years) followed for 4 years after screening with Hemoccult II testing and retrospective analyses of patients with known colorectal carcinoma or polyps or both who had Hemoccult II testing within 2 years of diagnosis. Setting: A large, health maintenance organization practice. Measurements and Main Results: In the prospective analysis, the sensitivity of Hemoccult II was 50% for colorectal carcinoma diagnosed within 1 year of testing, 43% within 2 years, and 25% within 4 years. For polyps, sensitivity was 36% at 1 year, 28% at 2 years, and 17% at 4 years. Specificity was 99%. The predictive value of a positive test for colorectal carcinoma was 8% at 1 year, 10% at 2 years, and 11 % at 4 years. On the basis of the retrospective analyses, the sensitivity of Hemoccult II for colorectal carcinoma diagnosed within 1 year of testing was 66% and was 61% within 2 years. Many of these patients had symptoms when tested. Conclusions: An asymptomatic patient age 45 or older with a positive Hemoccult II test has about a chance of 1 in 10 for having colorectal carcinoma and a 1-in-3 chance of having either a colorectal carcinoma or polyp: The same patient with a negative Hemoccult test has a 0.2% chance of having a colorectal carcinoma diagnosed within 2 years of testing and a 0.7% chance of having a polyp. Within 4 years of testing the chance increases to 0.5% for colorectal carcinoma and 1.5% for polyps. If Hemoccult II slides are the only screening method used for detecting asymptomatic colorectal neoplasms, 50% to 60% of lesions will remain undetected. Clinical interpretation of Hemoccult screening requires appreciation of its limits as well as its benefits.