OBJECTIVE: To investigate "up-to-date" and "age-appropriate" indicators of preschool vaccination status and their implications for vaccination policy. METHODS: The authors analyzed medical records data from the Baltimore Immunization Study for 525 2-year-olds born from August 1988 through March 1989 to mothers living in low-income Census tracts of the city of Baltimore. RESULTS: While only 54% of 24-month-old children were up-to-date for the primary series, indicators of up-to-date coverage were consistently higher, by 37 or more percentage points, than corresponding age-appropriate indicators. Almost 80% of children who failed to receive the first dose of DTP or OPV age-appropriately failed to be up-to-date by 24 months of age for the primary series. CONCLUSIONS: Age-appropriate immunization indicators more accurately reflect adequacy of protection for preschoolers than up-to-date indicators at both the individual and population levels. Age-appropriate receipt of the first dose of DTP should be monitored to identify children likely to be underimmunized. Age-appropriate indicators should also be incorporated as vaccination coverage estimators in population-based surveys and as quality of care indicators for managed care organizations. These changes would require accurate dates for each vaccination and support the need to develop population-based registries.