The leukemic lymphoblasts or a patient expressed CD7, CD13, CD33, CD34, HLA-DR and cytoplasmic CD3epsilon. He was diagnosed with acute lymphoblastic leukemia (ALL), and successfully treated with a conventional chemotherapy for ALL. The disease relapsed three times, and the character of the cells gradually altered, i.e. CD-56 expression increased and CD13, CD7 and cCD3epsilon decreased. The phenotype of the relapsed ALL was. therefore, compatible with myeloid/natural killer cell precursor acute leukemia (M/NK-AL). Some of M/NK-AL may be closely related with T/myeloid-biphenotypic pro-T blasts, and both types of acute leukemia may develop a tendency to express myeloid antigens, and they may belong to the category of immature T lymphoid precursors. (C) 2002 Elsevier Science Ltd. All rights reserved.