It was well established that drugs played a role in the etiology of aplastic anemia, agranulocytosis, and thrombocytopenic purpura. In 1985, much of the evidence concerning the relation of exposure to drugs to the risk of the three dyscrasias was based on case reports. Such reports could sometimes be appropriate for raising hypotheses, but they rarely served to establish associations firmly. They could also be subject to selection bias due to a tendency to publish reports only when a dyscrasia followed the use of a drug already under suspicion, and could result in spurious and non-causal associations. Moreover, since denominator populations were not known, case reports could never provide quantitative estimates of associations, either in terms of their magnitude or of the incidence rates attributable to the specific exposures The sparse quantitative information that did exist was of variable quality because of methodological and other difficulties. And while overall incidence rates, within orders of magnitude, could be estimated for the three dyscrasias, there was virtually no acceptably reliable information available on the incidence rates due to specific drugs. Even for well known associations such as aplastic anemia with chloramphenicol, estimates of incidence, if given at all, were imprecise.