Although academic interest in the study of mobilities is on the increase, exactly what it takes and what it means for data to become mobile is seldom asked. This paper addresses that question for the case of digital medical images, more precisely mammograms (X-ray images of the breasts). It is argued that the kind of reasoning which treats mobility as a fixed asset of such images is problematic, because it obscures the particular perceptions, circumstances and practices that play a part in the accomplishment of medical images as mobile. The argument is based on ethnographic involvement with an e-Science/telemedicine research project aimed at demonstrating the benefits of a digital mammography database for breast cancer screening services, epidemiological research and radiology teaching in the UK. By focusing on the ways in which mammograms are re-presented as ‘mobile data’, and on how their movement is practically organized in the context of this project, the paper indicates a new direction for the sociological study of data mobility: one that understands the relationship between ‘data’ and ‘mobility’ as accomplished and emerging rather than fixed and inherent.