In Canada since 1993 the nation's passive system for reporting postimmunization adverse events has been supplemented by the active surveillance of inpatients at 10 to 12 pediatric referral centers, a system referred to as the Immunization Monitoring Program, Active. Participating centers are located from coast-to-coast and receive referrals from every province and territory. Approximately 20 percent of the population aged 0 to 12 years lives in the immediate vicinity of these centers. Nurse monitors at each center search for numerous target conditions, including postimmunization adverse events and vaccine-preventable infections. Vaccine safety observations have included (1) a substantial decrease in the risk of the development of febrile seizures and hypotonic-hyporesponsive episodes since the country switched from whole-cell to acellular pertussis-containing vaccines, (2) no evidence for encephalopathy resulting from the latter vaccines, (3) a generally benign outcome with postimmunization thrombocytopenia cases, and (4) an unexpectedly high rate of disseminated bacille Calmette-Guérin infections among aboriginal infants. Concomitant disease surveillance has been important for sustaining the surveillance system because few postimmunization adverse events require hospital admission.