The Leukemia Intergroup Study has treated 110 patients with acute nonlymphocytic leukaemia with ‘high dose’ cytosine arabinoside remission induction therapy and studied the factors which were related to the outcome of therapy. With respect to death during remission induction therapy, only patient age was of prognostic significance. Treatment failure due to resistant leukaemia was associated with a high pretherapy leukaemic cell mass, the presence of few cells in S phase, and insensitivity of DNA synthesis to cytosine arabinoside. If, after 6 d of therapy, more than 40% of the marrow cells were leukaemic, the patient almost invariably failed to enter complete remission because of persistent leukaemia. Simultaneous consideration of the pretherapy labelling index and the per cent abnormal cells in the day 6 marrow permitted a distinction to be made between almost all patients who would enter remission or fail therapy because of persistent leukaemia.