Epidemiological data show that the incidence of aplastic anemia is a least 4-5 times as high in the far East as in the West. The reason seems to be complex--a number of exogenic factors act against a background of genetic differences in susceptibility. A Swedish study of aplastic anemia shows the overall incidence to be 13 cases per million inhabitants and year, but also that the incidence varies greatly with age (range 4-61 cases per million). Drugs are the single most important aetiologic factor. Chloramphenicol is the most common offender, but in many countries it now has been replaced by the anti-inflammatory agents phenylbutazone and oxyphenbutazone. In Sweden the latter also have disappeared and sulfonamide preparations dominate among drugs causing aplasia.