The advanced stage of most colorectal cancers at the time of presentation is largely responsible for the poor prognosis of the majority of patients with this tumour. Symptoms attributable to colorectal cancer are nonspecific and there is often considerable delay between their onset and diagnosis. By offering faecal occult blood (FOB) tests to patients presenting to their family doctors with abdominal symptoms the delay in diagnosis was significantly reduced but the pathological stage of the cancers was similar to that of a control group managed by their family doctors in their standard manner. In order to detect cancers at an earlier, asymptomatic stage a randomised control trial of FOB testing was instituted. A total of 26,975 individuals from the general population between the ages of 45 and 75 years were randomised to receive FOB tests or to be controls. 4,917 (36%) completed the tests and a total of 18 cancers were detected of which 13 (72%) were confined to the bowel wall. At rescreen a further six cancers (3 stage A) were detected by FOB tests. In the control group 20 cancers were diagnosed at two years of which only one (5%) was stage A. Asymptomatic screening showed a significant increase in the proportion of early stage tumours but the effect on mortality will await long term follow up.