Colorectal cancer mortality can be reduced by repeated faecal occult blood (FOB) testing followed by colonoscopy for test positives. The object of this report is to describe how to launch a new screening programme in such a way that its effectiveness can be reliably evaluated. The programme is based on gradual expansion over time with individual-level randomization into screening or control arms among a target population aged 60-69 years in Finland. The target population will be sampled from the population register for invitees and controls by municipality and by birth cohort. The non-invited controls will gradually be screened only after the six-year implementation period. After 10 years, the programme covers the entire target population. The effects of screening will be evaluated, comparing the incidence of and mortality from colorectal cancer in those invited to screening with controls. The primary screening test is a biannual guaiac-based FOB test with three test cards for consecutive samples. In September-December 2004, around 5000 test-kits were sent to 22 piloting municipalities. In 2005, the programme expands both among municipalities and the target population, resulting in nearly 20,000 individual requests. The implementation of colorectal cancer screening in Finland in this way meets the criteria for a randomized controlled trial and the requirements for a public health programme. It allows unbiased research data to be collected while introducing the programme and may set an example for the introduction of all national screening programmes.