The incidence of aplastic anemia was estimated in a 4-year study conducted in Israel and seven areas in Europe. Strict definition criteria were used, and all data and bone marrow specimens were reviewed by a panel of experienced hematologists. Complete ascertainment of cases was attempted by establishing a telephone network including all relevant hospitals in the study areas. The incidence of aplastic anemia was estimated to be less than three per million per year, a figure lower than previously reported. The most likely explanation for the differences among studies is variation in diagnostic criteria, which might lead to an overestimation of the incidence in some previous studies.