Four randomized controlled trials have demonstrated the efficacy of screening using a guaiac faecal occult blood test (gFOBT) on colorectal cancer (CRC) mortality. Whether their results are transposable to the real world is unknown. This study aimed to assess the determinants of participation in the first four rounds of the CRC screening programme using a gFOBT implemented since 2003 in the Haut-Rhin (Alsace) part of the French national programme.