Aplastic anemia, a form of bone marrow failure, is a disease of largely unknown cause, except for the rare situations involving high doses of radiation. Many agents have been mentioned in case reports to be associated with the development of aplastic anemia and include drugs, chemicals, pesticides, radiation, and viral infection. Of these, chloramphenicol, phenylbutazone, and insecticides are most frequently reported. In the United States and other developed countries the disease prevalence is so low that too few cases occur to make an epidemiological study in a single city logistically feasible. The Bangkok hospitalization rate for aplastic anemia was sufficient to conduct a case-control study. Also, a study conducted in this locale provided information on drugs and chemicals not commonly used in developed countries.