The visibility of microcalcifications in mammography is limited, inter alia, by the dimensions of the focal spot of the x-ray tube. The user should be able to evaluate a given nominal value of the focal spot and to assess the effective size of the focal spot in his x-ray tube. Determination of the resolution with the help of the mammographic phantom suggested by the German DIN standards specifications requires special care. In the framework of the German Mammography Study, 45 mammography devices at different locations were tested with regard to their focal spot sizes. All devices complied with the acceptance test without any adverse comment, although--if judged objectively--only half of them have the resolution demanded by DIN standards. Consequently, in most of the cases the users are unaware of this, and the negative effects on the visibility of possibly present microcalcifications are often underestimated. The diagnostic importance of microcalcifications for the detection of breast cancer suggests, however, to abstain from devices which are only of limited usefulness in relevant mammography screening.