A CDC program goal for 2010 is to achieve >95% participation in an immunization information system (IIS) (defined as having two or more recorded vaccinations) among children aged <6 years. IISs, previously known as immunization registries, are confidential, computerized information systems that collect and consolidate vaccination data from multiple health-care providers, generate reminder and recall notifications, and assess vaccination coverage within a defined geographic area. Most IISs have additional capabilities, such as vaccine management, adverse event reporting, maintenance of birth-to-death vaccination histories, and interoperability with other health information systems. IISs can provide accurate data on which to make informed vaccination decisions and better protect against vaccine-preventable diseases. This report highlights selected data from CDC's 2006 Immunization Information System Annual Report (IISAR), a survey of grantees in 50 states, five cities, and the District of Columbia that receive funding under the Public Health Service Act. The data indicated that 65% of all U.S. children aged <6 years (approximately 15 million children) participated in an IIS in 2006, an increase from 56% in 2005. The majority of grantees (70%) reported that their IISs have the capacity to track vaccinations for persons of all ages. Data on vaccinations were entered within 30 days of vaccine administration for 69% of children aged <6 years. However, results for certain data completeness measures were low. These findings underscore the need to continue to address challenges to full IIS participation and ensure high-quality information.