The objective of the study is to analyse individual women's participation patterns in mammography screening in Denmark. The study is set in the capital of Copenhagen and the county of Fyn representing around 95,000 women aged 50-69. The Central Population Register (CPR) was used to define the total target group, and supply information on migrations and deaths. Invitation and participation data came from the mammography screening programmes in Copenhagen (1991-1999) and Fyn (1993-2001), containing personal identification number, data on invitation date, participation and examination date for each screening round. In Copenhagen the coverage went from 70.5% in the first round to 63.1% in the fourth round, and the equivalent data for Fyn is 84.6% in the first round and 82.8% in the fourth round. Of the women eligible for at least three invitation rounds, 52.6% in Copenhagen and 76.4% in Fyn were faithful users, i.e. had participated in all screenings they were invited to. The conclusion is that the programme participation rates tend to overestimate the protection of the individual women covered by the programme. Behind the urban-rural gradient in programme participation is an even greater gradient in programme protection.