This is the first study to focus on Medicare mammography rescreening using a relatively large population of older women over a long followup period. To assess correlates of regular mammography, we followed all women age 65 or over enrolled continuously in Medicare fee-for-service (FFS) (n = 515,746) over a 7-year period. Data were drawn from the CMS claims data for the period 1992-1998. Irregular mammography intervals were more commonly found among vulnerable Medicare subpopulations--women who were older, minority, living in low income and lower education areas, and who were enrolled in both Medicare and Medicaid. Health care providers must communicate clearly to older women the breast cancer rescreening message: Not just once, but for a lifetime.