Bone marrow aspirates from 60 patients with acute myeloid leukemia (AML) were investigated using 95% ethanol fixation Papanicolaou stained preparations. The blasts were grouped into those with a clear halo around nucleoli (BCHN) and those without a clear halo. The patients were classified into three groups according to the degree of persistent BCHN at the end of induction therapy: group 1, no BCHN; group 2, less than 1% BCHN; and group 3, 1% or more BCHN. All patients in groups 1 (17 cases) and 2 (12 cases), and 12 of 31 cases in group 3 achieved complete remission (CR). Of 17 patients in group 1, two underwent bone marrow transplantation and two died from infection. Of the 37 patients who achieved CR, relapse was observed in two of 13 patients in group 1, and in all patients in groups 2 and 3. As to the patients treated with N4-behenoyl-1-beta-D-arabinofuranosyl-cytosine + daunorubicin + 6-mercaptopurine + prednisolone (BHAC-DMP) protocol, the percentages and number of BCHN at the diagnosis of AML in group 1 were significantly lower than those of groups 2 and 3. The percentage and number of BCHN at the diagnosis of AML were significant factors for the achievement of CR and for the prediction of long-term outcome. The reduction of BCHN to less than 1% at the end of induction therapy is a good indicator for the achievement of CR, and the disappearance of BCHN is a useful target for a long-lasting first CR; conversely, the persistence of BCHN is a major adverse factor for relapse.