The aim of this study was to determine the diagnostic value of rectal bleeding for distal colorectal cancer (CRC), or large (> or =10 mm) adenomas among an average-risk population. A cross-sectional survey was conducted among individuals aged 55-64 years, who attended sigmoidoscopy (FS) screening in the context of a multicentre randomised trial of FS screening for CRC. Sensitivity, specificity and positive predictive value (PPV) of rectal bleeding for large distal adenomas or CRC were calculated. Rectal bleeding was reported by 8.8% of 8507 patients examined (15% of those with large adenomas and 29% of those with CRC). The risk of CRC was increased when bleeding was associated with an altered bowel habit: odds ratio (OR)=10.42; 95% Confidence Interval (CI): 4.08-26.59; the corresponding OR for isolated bleeding was 5.29 (95% CI: 2.28-12.30). Rectal bleeding carries an increased risk of distal neoplastic lesions. However, most lesions are detected among asymptomatic subjects. This finding suggests that screening represents the optimal strategy to detect CRC or large adenomas in the distal colon in the targeted age range.