Colonoscopy is the established method of surveillance of subjects at high risk of developing colorectal neoplasia. Its role in the surveillance of a population at moderate risk is less clear, however, as the procedure is expensive, time consuming and occasionally hazardous. The aim of this study was to estimate by case-control methods the effect of faecal occult blood (FOB) screening on colorectal cancer (CRC) mortality in a population at moderate risk of developing CRC. Screening by FOB testing prior to diagnosis in patients over the age of 45 years who died of CRC diagnosed in 1989-1998 was compared with screening in controls matched with the case for age and sex. Information about episodes of FOB testing and potential confounders was obtained from the data collection system of the screening programme. Cases were less likely than controls to have ever been screened, with an odds ratio of 0.64 (95% confidence interval 0.34-1.15) for exposure to at least one FOB testing. There was no significant difference between the sub-groups according to gender, age at diagnosis or location of the cancer. The inverse association between screening for faecal occult blood and fatal colorectal cancer suggests that screening in a population at moderate risk of CRC can reduce mortality from CRC in this group.