CONTEXT: Vaccination coverage rates can be improved through the application of complete and accurate immunization information systems (IISs). OBJECTIVE: Evaluate the completeness and accuracy of Wisconsin's IIS- the Wisconsin Immunization Registry (WIR). DESIGN: Cross-sectional evaluation- comparing vaccination medical records (MRs) from provider clinics with WIR records. PARTICIPANTS: Medical records of patients born during 2009 were randomly selected from 251 Wisconsin clinics associated with the Vaccines for Children Program. MAIN OUTCOME MEASURES: Completeness: percentage of patients with client records in the WIR- percentage of patients up-to-date (%UTD) with the 4:3:1:3:3:1:4 vaccination series- and percentage of patients' MR vaccinations matched by administration date (