Since the days when chloramphenicol was more commonly used, it has been recognized that many antimicrobial drug are associated with severe blood dyscrasias, such as aplastic anemia, neutropenia, agranulocytosis, throm-bocytopenia, and hemolytic anemia. Information on this association has come predominantly from case series and hospital surveys (38^. Some evidence can be extracted from population-based studies that have focused on aplastic anemia and agranulocytosis and their association with many drugs, including antimicrobial drugs. The incidence rates of blood dyscrasias in the general population have been estimated in a cohort study with a nested case-control analysis, using data from a General Practice Research Database in Spain.