To investigate the origin of cells observed in acute myelomonocytic leukemia with eosinophilia (M4E₀), we carried out morphological and cytogenetic studies of colonies derived from leukemic precursors. Marrow cells from a patient with M4E₀ at second relapse revealed an abnormal karyotype; 47, XY, +8, inv(16)(p13q22), 17p-. Cytogenetic studies of leukemic colonies showed the same karyotypic abnormality. The primary and secondary colonies consisted of monocytes, eosinophils and basophils. These results indicated that the neoplastic process in this patient originated in a stem cell capable of differentiation into at least monocytes, eosinophils and basophils.