In Manitoba, Canada, a centralized, computerized childhood immunization monitoring system serves a population with insured medical coverage; each individual has a unique identification number. All physicians bill a single payer, and specific billing codes are used to identify immunizations given by physicians and by public health nurses. Together with dates of service, this information is used to construct immunizations-to-date and age-at-immunization profiles for individuals and groups. Reminders seeking missing information are built into the system and executed at strategic ages. The quality of input data has been assessed as high. The approach could be used readily by health maintenance organizations in the United States.