The design was that of a case-control study. All hospitals with at least 100 beds in geographically defined areas of eastern Massachusetts and Rhode Island were enrolled. All cases of aplastic anemia and agranulocytosis were identified prospectively for the purpose of estimating incidence rates. A proportion of cases of thrombocytopenic purpura were identified but incidence rates were not estimated. The eligibility of cases was determined by a committee of hematologists, according to strict diagnostic criteria. All identified cases of the dyscrasias, suitable hospital controls, and, in Massachusetts, neighbor controls were interviewed. Information was obtained on drug use, occupational and chemical exposure, personal data, and relevant medical history. The data were used to quantify known associations between the three dyscrasias and drug use, to identify and quantify previously unsuspected associations, and to document absence of associations for commonly used drugs. Incidence rates of aplastic anemia and agranulocytosis attributable to specific drugs and classes of drugs were estimated.