We developed a simulated immunization registry to assess the impact on the perceived immunization status in a population-based sample of 2-year-olds living in Olmsted County, MN, in 1995. We compiled records of all immunizations by abstracting immunization data from all medical care facilities in the county. The data collected from each facility were analyzed separately to provide the immunization rate as perceived by each facility. This perceived rate was compared to the rate obtained by combining all recorded immunizations from all facilities (simulated registry). Information on children not receiving any carefrom facilities in Olmsted County was compiled from birth certificate data and community school lists. Data from the simulated registry indicated that 69.1% of all children in Olmsted County with medical records were up-to-date on their immunizations by 20 months of age. By 24 months, this increased to 74.2%. The immunization rate of 24-month-old children recorded at individual healthcare facilities in Olmsted County ranged from 24.3% to 79.5%. The addition of data from the simulated registry increased the immunization rate at each site: a 27.7% relative increase in the site with the lowest recorded immunization rate, a 14.0% increase in the site with the intermediate immunization rate, and a 6.9% increase in the site with the highest internally perceived immunization rate. The registry also identified excess immunizations in 5% of the county's 2-year-olds. Each healthcare facility in this community gained an immediate benefit from the development of a simulated immunization registry. This immediate improvement in one quality-of-care measure (up-to-date immunization rate) should be factored into the cost/benefit assessment of immunization registries.