The bone marrow chromosomes of 25 children with acute lymphoblastic leukaemia (ALL) were examined at diagnosis before treatment, during remission, and in 12 cases, also during relapse. Follow-up was for at least six years. At diagnosis, 17 patients had a major population of chromosomally abnormal cells and of these 11 had identifiable clones. The commonest abnormality was hyperdiploidy. Eight patients had predominantly normal cells, but four of these had a minor abnormal clone. In remission, some samples were completely normal but, when pooled, remission samples had a minor population of chromosomally aberrant cells which were rarely clonal. The incidence of structural abnormalities was the same in patients who ultimately relapsed and those who remained in first remission at the end of the study, but the presence of hyperdiploid cells and/or clones in remission was more frequently associated with subsequent relapse. Relapse patterns were of two kinds: in three patients there was a return of the chromosomal abnormalities seen at diagnosis; in six others, chromosomal features in relapse were distinct from those at diagnosis. It is suggested that relapse associated with distinct chromosomal features may represent malignant transformation of a previously unaffected cell line. While chromosomal abnormalities seen prior to treatment can be related to the leukaemic event alone, abnormalities seen in remission and in relapse may result partly from drug and X-ray treatment. The relative importance of treatment and other factors to chromosomal change in ALL is discussed.