The assumption that asymptomatic colorectal cancers bleed provides the rationale for widespread stool screening. The authors studied 12 patients with unoperated colorectal cancer but without colorectal symptoms and six healthy volunteers as laboratory controls. All stools were collected for 2 weeks and analyzed by the HemoQuant and Hemoccult tests. In controls, the mean HemoQuant value was 0.7 mg hemoglobin (Hb)/g stool (range, 0.1-1.8) and all stools were Hemoccult-negative. In cancer patients, the mean HemoQuant was 3.3 mg Hb/g (range, 0.3-13.2); stools were within the normal HemoQuant range (less than 2 mg Hb/g) in 38% and negative by Hemoccult in 70%. The mean cancer detection rate testing a single stool per patient was 57% for HemoQuant and 25% for Hemoccult (P less than 0.001). The detection rate rose testing multiple stools and was maximal with five stools at 83% for HemoQuant compared to 31% for Hemoccult (P less than 0.001). The authors conclude that fecal blood levels are commonly normal with asymptomatic colorectal cancer. Although higher with HemoQuant than Hemoccult, cancer detection rates by fecal blood testing appear to be lower than previously reported.