The participation rate in organized breast cancer screening in France is lower than recommended. Non-participants either use opportunistic screening or do not use either screening modality. To assess any differences in perceptions, attitudes and knowledge related to breast cancer screening between users of opportunistic screening and non-users of any screening mammograms and to identify potential barriers to participation in organized screening. Six focus groups were conducted in May 2010 with 34 French non-participants in organized screening, 15 who used opportunistic screening (OpS group) and 19 who used no screening (NoS group). The guide used for both groups explored perceptions and attitudes related to health, cancer and screening; perceptions of femininity; and knowledge about breast cancer screening. Thematic content analysis was performed. Perceptions, attitudes and knowledge differed between the two groups. Women in the OpS group perceived a high susceptibility to breast cancer, visited their gynaecologist regularly, were unfamiliar with organized screening modalities and had doubts about its quality. NoS women had very high- or low-perceived susceptibility to breast cancer, knew about screening modalities, had doubts about its usefulness and expressed negative opinions of mammograms. Differences in perceptions and attitudes related to breast cancer screening partially explain why some women choose opportunistic screening or no screening. General practitioners and gynaecologists are in a unique position to provide individually tailored preventative messages to improve participation in organized screening.