Our objective was to use individual data on socio-demographic characteristics to identify predictors of participation in mammography screening and control to what extent they can explain the regional difference. We used data from mammography screening programmes in Copenhagen, 1991-1999, and Funen, 1993-2001, Denmark. Target groups were identified from the Population Register, screening data came from the health authority, and socio-demographic data from Statistics Denmark. Included were women eligible for at least 3 screens. The crude RR of never use versus always use was 3.21 (95% CI, 3.07-3.35) for Copenhagen versus Funen, and the adjusted RR was 2.55 (95% CI, 2.43-2.67). The adjusted RR for never use among women without contact to a primary care physician was 2.50 (95% CI, 2.31-2.71) and 2.89 (95% CI, 2.66-3.14), and for women without dental care 2.94 (95% CI, 2.77-3.12) and 2.88 (95% CI, 2.68-3.10) for Copenhagen and Funen, respectively. Other important predictive factors for nonparticipation were not being married and not being Danish. In conclusion, to enhance participation in mammography screening programmes special attention needs to be given to women not using other primary health care services. All women in Copenhagen, irrespective of their socio-demographic characteristics, had low participation. Screening programmes have to find ways to handle this urbanity factor.