Background: In Denmark, colorectal cancer (CRC) is the third most frequent cancer. Randomised trials have shown that guaiac faecal occult blood test (gFOBT) screening can reduce CRC mortality, but a recent large randomised study from Finland did not find any effect. A feasibility study was carried out in Denmark in 2005-2006 where residents aged 50-74 years in 2 Danish counties were invited once to participate in gFOBT screening. We used the unique Danish registers to assess the impact of gFOBT screening in this group on CRC incidence and mortality. Methods: In this cohort study, we followed a group comprising 166 277 individuals invited to screening and a reference group comprising the remaining 1 240 348 Danes of the same age. We linked the Danish population and health service registers to obtain information about colonoscopies, polypectomies, incident CRC and cause of death. Results: After a median follow-up time of 8.9 years, the CRC mortality was significantly lower in the screening group than in the reference group with an adjusted HR (aHR) of 0.92 (95% CI 0.86 to 0.99), while the aHR for all-cause mortality was 0.95 (95% CI 0.94 to 0.96). For screening participants, the aHR for CRC mortality and all-cause mortality was 0.72 (0.64 to 0.80) and 0.59 (0.57 to 0.60), respectively. Conclusions: About 10 years after a single round of gFOBT screening, we found a significant 8% deficit in CRC mortality in the screening group compared with other Danes. We found almost the same deficit in allcause mortality, and on this basis, it is not possible to conclude that one screening round had an effect on CRC mortality. Our study indicated that close monitoring of the outcome of CRC screening is warranted.