Background. The effectiveness of mammography screening could be improved if factors that influence nonattendance were better understood. Methods. We examined attitudes, beliefs, and knowledge in relation to nonattendance in a population-based mammography screening program, using a case-control design. Data were collected from November 1997 to March 1998 through telephone interviews with 434 nonattenders and 515 attenders identified in a population-based mammography register in central Sweden. The questions asked drew primarily upon the components constituting the Health Belief Model. Results. Multivariate analysis showed that nonattendance was most common among women within the highest quartile of perceived emotional barriers, compared to women within the lowest quartile (OR = 4.81; 95% CI 2.96-7.82). Women who worried most about breast cancer were more likely to attend than those who worried least (OR = 0.09; 95% CI 0.02-0.31). Women with the highest scores of perceived benefits were more likely to attend than women with the lowest ones (OR = 0.35; 95% CI 0.08-0.75). Other factors associated with nonattendance were less knowledge about mammography and breast cancer, lack of advice from a health professional to participate, and very poor trust in health care. Conclusions, Our findings suggest that increased participation in outreach mammography screening programs can be achieved through enhancement of breast cancer awareness and possibly by reducing some of the modifiable barriers.