An in vitro BrDU chromosome labelling technique has been used for analysing the growth pattern and incidence of sister chromatid exchanges (SCE) in the bone marrow cells of twelve patients with acute lymphoblastic leukaemia (ALL): six with only diploid cells and six with aneuploid cells, containing various numerical and structural chromosome abnormalities, with or without concurrent diploid cells. There was no clear-cut karyotypic relevance to the growth pattern or SCE incidence in such cells. An increased SCE incidence was observed during or immediately after treatment with certain chemotherapeutic regimens. Serial analyses of the growth pattern of cells at diagnosis, at the active stage of the ALL, and during induction of remission, revealed a distinct difference in cell cycle time between leukaemic cells and non-leukaemic ones, the former having a considerably longer cell cycle time than the latter. However, no clear change in the growth pattern of such cells was observed as a function of chemotherapy. Our data also suggest that the growth pattern and SCE incidence in ALL cells are not related to the age of patients and the type of cells involved, as defined by cell surface marker studies.