All aplastic anemia cases in metropolitan Bangkok were located and verified in the forty hospitals that agreed to participate in the study. Incidence rates were calculated by age group and sex. In the case-control study, interviews were used to obtain comprehensive histories of drug, chemical, pesticide, radiation, viral infections, and other exposures, along with relevant demographic and medical history information in order to examine the association between and test hypotheses regarding suspected hazards and to estimate the risks of developing aplastic anemia. The basic epidemiologic features of aplastic anemia were described by characterizing these patients in terms of standard demographic variables, age, race, sex, and medical history. The degree of association between viral hepatitis and the development of aplastic anemia was estimated.