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An apparent cluster of four aplastic anemia (AA) cases in teenagers residing in a small South Carolina town was further investigated. Incidence of AA in all age groups in a surrounding three county area (TCA) over a 12-year time interval was determined and compared with AA incidence rates in Baltimore, representing the only known population based United States incidence data. The same general age-specific incidence pattern (based on 27 cases in the TCA and 118 in Baltimore) was found in the two areas, both overall and for the four race-sex groups. Although based on small numbers, nonwhite average annual age-adjusted rates for males and females were higher in the TCA (6.8 and 13.7 per million) than in Baltimore (4.7 and 7.3). For whites, TCA rates were 11.7 and 5.4 (for males and females) and Baltimore rates were 7.1 and 5.4. The differences for non-whites in the two areas may indicate a greater prevalence of risk factors for AA in the TCA than in Baltimore, but the small numbers of cases and the lack of comparable data from other areas of the country, together with the possibility of misdiagnosis of the disease, make definitive conclusions impossible.