To protect the citizens of the United States from the use of dangerous biological agents, the Center for Disease Control and Prevention (CDC) has been actively preparing to deal with the consequences of such an attack. Their plans include the deployment of mass immunization clinics to handle postevent vaccinations. As part of the planning efforts by the Western New York Public Health Alliance, a Web-based electronic patient registration and tracking system was developed and tested at a recent trial smallpox vaccination clinic. Initial goals were to determine what the pitfalls and benefits of using such a system might be in comparison to other methods of data collection. This exercise proved that use of an electronic system capable of scanning two-dimensional bar codes was superior to both paper-based and optical character recognition (OCR) methods of data collection and management. Major improvements in speed and/or accuracy were evident in all areas of the clinic, especially in patient registration, vaccine tracking and postclinic data analysis.