Aplastic anemia is believed to be a disease of high incidence in the Orient. However, no population-based study yet exists to verify this belief. It is proposed to conduct such an incidence study in metropolitan Bangkok in a manner which will yield valid comparisons with the few western studies of aplastic anemia incidence. Aplastic anemia is a disease of largely unknown etiology. While some evidence exists that occasional cases may be the result of short and long term side effects of a certain drugs and chemicals, such associations have not been consistently observed. In the United States and other developed countries the disease prevalence is low to such a degree that too few cases cluster in space and time to render epidemiologic study in a single city logistically feasible. The Bangkok hospitalization rate for aplastic anemia is sufficient to conduct a sound case-control study of this disease. Furthermore, a study conducted in this locale would provide information on drugs and chemicals not commonly in use in developed countries. Therefore, the second component of this proposal is a study of the epidemiology of aplastic anemia using the case-control design. Cases will be patients with aplastic anemia from Siriraj Hospital and all other hospitals in metropolitan Bangkok. Controls will be selected from patients with trauma and other acute conditions from the same hospital, broadly matched with cases for age, sex and the time of hospitalization. There will be 100 cases and 400 controls. Each subject will be interviewed using a standardized questionnaire. Data will be obtained on exposures to drugs, pesticides, chemicals, radiation, and viral infections. In addition, the proportions having evidence of recent hepatitis for cases and appropriate controls will be compared.