It has been shown previously that it is difficult for a general practitioner to predict anal vs. colorectal sources of bleeding in patients presenting with rectal bleeding. The aim of the present study was to determine whether there are any aspects of such a patient's history or clinical features that strongly indicate bleeding from a colorectal cancer or polyp. One hundred forty-five consecutive patients, aged 40 years and older, who had complained of rectal bleeding to a general practitioner, were referred to a specialist for full colonic investigation. Among 15 symptoms and clinical features examined, few had any statistically significant association with the source of bleeding. There was an elevated probability of colorectal cancer (21 percent) in patients who had seen blood mixed with feces. Most bowel symptoms and clinical features are not helpful in deciding whether to proceed with full colorectal assessment in patients aged 40 and older who have rectal bleeding of recent onset.