OBJECTIVES: Immunization information systems (IIS) consolidate immunization information from participating providers within defined geographic areas. State laws regarding IIS reporting requirements vary, and impact on the completeness of IIS vaccination information is unclear. The objectives of this systematic review were to 1) compare completeness of IIS data to medical records, 2) compare completeness of IIS data to personal records, and 3) explore characteristics of IIS policy that may influence completeness. METHODS: Databases searched included Medline, PsychINFO, and CINAHL. Search terms used were ((immunization registry) OR (immunization information system)) AND ((accuracy) OR (complete) OR (quality)) AND ((medical record) OR (EMR) OR (personal record) OR (self report)). Grey literature and reference lists were hand-searched, extracting articles previously unidentified. Studies of registries outside the U.S. or not including a comparison data source were excluded. Studies selected included those measuring completeness through percentage of individuals up to date or receiving a particular vaccine. RESULTS: CONCLUSIONS: More research is needed to assess the completeness of IIS data compared to other sources of immunization information. Factors other than legislation may play a role in IIS data completeness and need to be considered.