Agranulocytosis and aplastic anemia are rare but serious blood diseases that can be caused by drugs and other environmental factors. In 1980, there were only limited quantitative estimates of the risk between various exposures, particularly drugs, and the two dyscrasias. The data were collected in the IAAAS, a multicenter population-based case-control study conducted from 1980 to 1986 in eight regions. The study was funded by a pharmaceutical company to investigate a single hypothesis, but information was obtained on all drugs and on other factors, enabling a general investigation of the dyscrasias.