OBJECTIVES: Recent guidelines recommend colorectal cancer screening of average-risk population. Our objective was to estimate the resources needed (costs and colonoscopies) to implement a population-based colorectal cancer screening program. METHODS: A discrete-event simulation model was built to represent a colorectal cancer screening program for a target population of 100,000 women and men aged 50 to 69 years. The conceptual model for the screening process was based on the European Guidelines, which recommend biennial screening with immunochemical faecal occult blood test (iFOBT) and colonoscopy for positives of iFOBT. Follow-up after adenoma removal differed according to findings of each colonoscopy classified by risk of adenomas. Parameters for initial screening were estimated from the areas corresponding to Hospital del Mar within the Colorectal Cancer Screening Program of Barcelona. Parameters for successive screenings were obtained from the literature. A 20-year horizon was simulated. The model included the population ageing. Costs included, lettering, personnel, iFOBT and colonoscopy. RESULTS: Annual cost of screening a target population of 100,000 people varied from 899 thousand euro the first simulated year to 1,322 thousand euro the 20th year. The number of screening colonoscopies varied from 1,659 the first year to 1,790 the 20th year. Colonoscopies for following-up screening findings should be added to this numbers, starting in the second year: from 245 to 1,371 follow-up colonoscopies the 20thyear. CONCLUSIONS: Implementing a population-based colorectal cancer screening program following the European guidelines represents a cost of around 1 million euro by year the first five years for a target population of 100,000 inhabitants. The cost increases during the following 15 years until 1322 thousand euro. An important consequence of screening is the increase in the demand of confirmatory and follow-up colonoscopies related to the findings generated by the program. The demand of follow-up colonoscopies is directly related to the intensity of follow-up through colonoscopy.