This paper describes the prototype for a Europe-wide distributed database of mammograms entitled MammoGrid, which was developed as part of an EU-funded project. The MammoGrid database appears to the user to be a single database, but the mammograms that comprise it are in fact retained and curated in the centres that generated them. Linked to each image is a potentially large and expandable set of patient information, known as metadata. Transmission of mammograms and metadata is secure, and a data acquisition system has been developed to upload and download mammograms from the distributed database, and then annotate them, rewriting the annotations to the database. The user can be anywhere in the world, but access rights can be applied. The paper aims to raise awareness among radiologists of the potential of emerging "grid" technology ("the second-generation Internet").