Abstract: Background & study aims : Investigation of the first participation rate and follow-up results of the Flemish colorectal cancer screening program. Patients & methods : In 2013 five age cohorts with an even age between 66 and 74 year old (n = 243 335) were invited by mail to return a completed iFOBT. Participants who tested positive (>= 75ng/ml) were referred to a follow-up colonoscopy. Results : Participation rate was 48.4% (n = 117 774). Overall positivity rate was 10.1%, and 78.1% of those tested positive underwent a colonoscopy. The positive predictive value of colonoscopy for CRC was 8.2%, for advanced adenoma 16.9% and for non-advanced adenoma 36.5%. Conclusion : Based on the EU-guidelines 35% was expected as participation for a first screening round, thus a participation rate of 48.4% is more than acceptable for a first screening year. The high positivity rate can partly be explained by including only the older ages in the start-up-period and by the first year of mass screening in Flanders.