In February of 2004, the New York City Department of Health and Mental Hygiene completed the integration of its childhood immunization and blood lead test registry databases, each containing over 2 million children. A modular approach was used to build a separate integrated system, called Master Child Index, to include all children in both the immunization and lead test registries. The principal challenge of this integration was to properly align records so that a child represented in one database is matched with the same child in the other database. To accomplish this task as well as to identify internal duplicate records within each database, an artificial intelligence record linkage system was created. The preliminary results show high rates of accurate merging of records both within and between the two databases. The 4,610,585 records contained in both databases before Master Child Index implementation consolidated into 2,977,290 records in the integrated system. The matching system eliminated 523,720 duplicate records within the two databases and matched and merged 1,109,575 records between the two databases. The Department of Health and Mental Hygiene plans to further develop the Master Child Index and use it as the department-wide, record-matching system.