Despite its proven efficacy in three randomized trials, the relevance of mass screening for colorectal cancer using the guaiac faecal occult blood test is still debated. The low sensitivity of the test and the poor participation rate, especially in France, are major obstacles to its effectiveness. The aim of our study was to characterize cancers occurring after a negative test and among non-participants in the screening programme organized in the French department of Calvados. Cancers in the negative test group had a later stage of extension than subjects testing positively but an earlier stage of extension than cancers in the reference group, which were not different from those of non-responders. The proportion of resection for non-responders was significantly lower than that for participants, whatever the test result (P < 0.001), and lower than that for reference subjects (P < 0.05). There was no difference in treatment between negative and positive responders. Negative responders did not have a delayed cancer diagnosis or a worse condition of treatment than people who were not screened. Low sensitivity reduced the efficacy of colorectal cancer screening but did not seem to increase the potential to do harm.