Summary. Transient pancytopenia preceded the onset of acute leukaemia in eight of 360 (2%) of children with acute lymphoblastic leukaemia (ALL) but did not occur in 70 cases of acute non-lymphoblastic leukaemia. The patients developed overt leukaemia within 5—38 weeks of first presentation with features of marrow failure. Immunological classification of blast cells was performed in six of the eight patients and all had the phenotype of common-ALL (c-ALL). We conclude that the syndrome of pre-leukaemia aplasia in childhood is a feature of c-ALL.