Three patients with aplastic anaemia following chloroquine therapy are described. In two, chloroquine had been administered in large doses over a long period. One of them subsequently developed acute myeloblastic leukaemia. The third received only a small dose and pancytopenia with aplasia followed three weeks after ingestion of the drug. The mechanism(s) of chloroquine-induced marrow injury is not known. A parallel to chloramphenicol-induced blood dyscrasias is drawn, but remains unproved.