There is growing evidence that faecal-occult-blood (FOB) screening may reduce colorectal cancer (CRC) mortality, but this reduction in CRC mortality has not been shown in an unselected population-based randomised controlled trial. The aim of this study was to assess the effect of FOB screening on CRC mortality in such a setting. Between February, 1981, and January, 1991, 152,850 people aged 45-74 years who lived in the Nottingham area of the UK were recruited to our study. Participants were randomly allocated FOB screening (76,466) or no screening (controls; 76,384). Controls were not told about the study and received no intervention. Screening-group participants were sent a Haemoccult FOB test kit with instructions from their family doctor. FOB tests were not rehydrated and dietary restrictions were imposed only for retesting borderline results. Individuals with negative FOB tests at the first screening, together with those who tested positive but in whom no neoplasia was found on colonoscopy, were invited to take part in further screening every 2 years. Screening was stopped in February, 1995, by which time screening-group participants had been offered FOB tests between three and six times. Screening-group participants who had a positive test were offered full colonoscopy. All participants were followed up until June, 1995. The primary outcome measure was CRC mortality. Of the 152,850 individuals recruited to the study, 2599 could not be traced or had emigrated and were excluded from the analysis. Thus, there were 75,253 participants in the screening group and 74,998 controls. 44,838 (59.6%) screening-group participants completed at least one screening. 28,720 (38.2%) of these individuals completed all the FOB tests they were offered and 16,118 (21.4%) completed at least one screening but not all the tests they were offered. 30,415 (40.4%) did not complete any test. Of 893 cancers (20% stage A) diagnosed in screening-group participants (CRC incidence of 1.49 per 1000 person-years), 236 (26.4%) were detected by FOB screening, 249 (27.9%) presented after a negative FOB test or investigation, and 400 (44.8%) presented in non-responders. The incidence of cancer in the control group (856 cases, 11% stage A) was 1.44 per 1000 person-years. Median follow-up was 7.8 years (range 4.5-14.5). 360 people died from CRC in the screening group compared with 420 in the control group-a 15% reduction in cumulative CRC mortality in the screening group (odds ratio=0.85 [95%; CI 0.74-0.98], p = 0.026). Our findings together with evidence from other trials suggest that consideration should be given to a national programme of FOB screening to reduce CRC mortality in the general population.