OBJECTIVES Tracking vaccine reactions and adverse events during a large-scale vaccination program such as the recent smallpox program or a pandemic flu outbreak will be a challenge. We report on vaccine reaction data collected using a novel telephone- and web-based electronic reporting system. The system was used to monitor vaccinees during the U.S. Army's smallpox vaccination campaign, which was part of the national program to prepare against biological attack. In addition, we report on the time course of events after smallpox vaccination based on the self-reported data and evaluate the validity and reliability of self-reported take information after smallpox vaccination. METHODS A prospective cohort of subjects receiving the smallpox vaccination volunteered to use an electronic monitoring system to track and report their vaccination reactions. RESULTS Users made 6.8 +/- 6.2 (mean +/- SD) reports using the electronic monitoring system. The sensitivity and positive predictive value of self-reported takes were high, 98.8% and 99.6%, respectively. The vaccination-site reactions progressed faster for revaccinees than first-time vaccinees. CONCLUSIONS Simple-to-use telephone/Internet-based technology allowed detailed self-recording of response to smallpox vaccination among outpatients. Self-reports on site appearance were sufficient to determine vaccine takes in most vaccinees. During a mass vaccination event, an electronic monitoring system could facilitate tracking of vaccine reactions, including providing an early warning system for adverse events, and might reduce the burden associated with follow-up visits with healthcare professionals.