With the collaboration of all coordinating physicians responsible for breast cancer screening in France, the authors studied factors related to personal history, mammographic characteristics prior to the diagnosis and the tumours of women with breast cancers larger than 20 mm. This retrospective study concerned 7,407 cancers larger than 20 mm diagnosed in the context of screening.Although criteria of tumour aggressiveness were more marked and technological factors were more commonly encountered, compliance with screening appears to be a major determinant in the discovery of these large tumours.It is essential for the population to take up these screening programmes in order to have an impact on the prognosis of breast cancer..