To assess the effectiveness of screening by faecal occult blood tests, 107,349 people without symptoms of colorectal disease identified from general practitioner records have been randomly allocated to test and control groups. 53,464 test subjects were invited to carry out the screening test; 27,651 (53%) of the 52,258 who received the tests did so. Further investigation of the 618 (2.3%) with positive tests showed 63 cancers (52% stage A) and 367 adenomas (266 subjects). Rescreening of subjects with negative results every 2 years (9510 first rescreen, 3639 second) has shown a significant fall in the rate of positive results (1.7% of 7344; 0.3% of 2906). Cancers have also been diagnosed in 20 subjects presenting in the interval between a negative test and rescreening, and in 83 non-responders. The incidence of cancer in the control group (123 subjects; 10.6% stage A) was 0.72 per 1000 person-years. Cancers detected by screening were at a less advanced pathological stage, but it is too early to show any effect of screening on mortality from colorectal cancer.