Abstract This study used data from the population database through which the province of Manitoba, Canada, administers its universal health insurance plan. Enrollment, hospitalization, and immunization files from children born in the 1987–1989 period were linked using the unique identification number assigned to each population member. Analysis of these linked data successfully identified serious potential adverse events in the first year of life and the timing of events around immunization. Not only is population-based active surveillance for immunization-related events feasible, but the techniques described, applied to years of data accumulated through surveillance, offer powerful research tools. Baseline population incidences of adverse events were calculated, temporal relationships between events and immunization assessed, and incidences for events showing true temporal associations determined. Eventual goals are the quantification of vaccine-related risk and the gathering of evidence concerning causal associations. The approach could be used readily by several other Canadian provinces and by health maintenance organizations in the United States.