A national mammography database was proposed, based on a centralized architecture for collecting, monitoring, and auditing mammography data. We have developed an alternative architecture relying on Internet-based distributed queries to heterogeneous databases. This architecture creates a "virtual repository", or a federated database which is constructed dynamically, for each query and makes use of data available in legacy systems. It allows the construction of custom-tailored databases at individual sites that can serve the dual purposes of providing data (a) to researchers through a common mammography repository and (b) to clinicians and administrators at participating institutions. We implemented this architecture in a prototype system at the Brigham and Women's Hospital to show its feasibility. Common queries are translated dynamically into database-specific queries, and the results are aggregated for immediate display or download by the user. Data reside in two different databases and consist of structured mammography reports, coded per BIRADS Standardized Mammography Lexicon, as well as pathology results. We prospectively collected data on 213 patients, and showed that our system can perform distributed queries effectively. We also implemented graphical exploratory analysis tools to allow visualization of results. Our findings indicate that the architecture is not only feasible, but also flexible and scaleable, constituting a good alternative to a national mammography database.