The attendance rate to population based primary screening colonoscopy within NordICC trial (The Nordic-European Initiative on Colorectal Cancer, NCT00883792) in Poland is roughly 33%. The aim of this study is to compare the effect of two different time schedules for sending invitations and reminding letters to a pre-specified colonoscopy appointment. 1,600 screening naive men and women 55 to 64 years of age are randomly assigned in a 1:1 ratio to receive invitation and reminder 6 and 3 weeks (SIX and THREE group) or 4 and 2 weeks (FOUR and TWO group) prior to pre-specified screening colonoscopy appointment date. Outcome measures are participation in screening colonoscopy within 6 months from the date of invitation, response and acceptance rate before sending a reminder. The study is powered to detect 7% difference in participation in screening colonoscopy.

Inclusion criteria:

Individuals aged 55-64 years living in Warsaw area randomly drawn from Population Registry and randomized to the screening arm of the NordICC trial.

Exclusion criteria:

• Resident abroad
• Return of unopened letter of invitation and/or reminder (address unknown)
• Message from neighbour/family/post office on death of screenee (not updated in Population Registry).

Exclusion criteria for colonoscopy:

• Individuals with previous colorectal surgery (resections, enterostomies)
• Individuals in need of long-lasting attention and nursing services (somatic or psychosocial, mental retardation).
• On-going cytotoxic treatment or radiotherapy for malignant disease
• Severe chronic (longer than trial duration) cardiac (NYHA III-IV)or lung disease
• Lifelong anticoagulant therapy with Warfarin
• A coronary event requiring hospitalization during the last 3 months
• A cerebrovascular event during the last 3 months