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Incidence rates for aplastic anemia (AA) so far have been unavailable for defined populations in the United States. A study was carried out in the Baltimore Standard Metropolitan Statistical Area (SMSA) to examine the incidence rates for aplastic anemia from 1970 through 1978. Cases of AA (N = 118) were identified through medical records (N = 77) in SMSA hospitals and death certificates (N = 41). All medical charts were reviewed by an oncologist-hematologist for validation purposes. Among whites, average annual age-adjusted incidence rates per million were higher in males (7.1) than in females (5.4), whereas in nonwhites, females had a higher rate (7.3) than males (4.7). None of the sex differences was statistically significant. Age-specific incidence rates were consistently low in young ages, with an exponential increase after age 40. Examination of time trends did not suggest changes in whites, although in blacks, mainly in males, there was a suggestion of a temporal increase. However, rates in blacks were based on small numbers, and trends were not statistically significant. An inconsistent sex differential, as well as the relative stability of rates over time at least in whites, suggests that although sexes may have different types of exposures, occupational exposures and changes in environmental factors over time cannot entirely explain the occurrence of AA in the population. In addition, the age pattern suggests that future studies should examine etiologic agents separately for the younger and the older subjects with AA.