Bone marrow aspirate was performed on day 4 of a 7-day induction chemotherapy regimen using daunorubicin, cytosine arabinoside and thioguanine in 42 patients with acute myeloid leukemia (AML). The mean percentage of remaining abnormal cells in day 4 bone marrow was significantly higher in patients having resistant disease (RD) than in patients entering complete remission (CR). Except for patients with FAB M3 AML, most patients who had more than 40% abnormal cells in their day 4 bone marrow had RD. In addition, patients entering CR despite having more than 40% abnormal cells remaining in their day 4 bone marrow seemed to have short CR.