Cytogenetic studies were performed in 18 consecutive children with acute nonlymphocytic leukemia (ANLL) between 1981 and 1983. Three children with acute myelomonocytic leukemia (AMMoL; M4, FAB classification) had the following unique bone marrow morphology and cytogenetic abnormality: eosinophilic precursors with dysplastic violaceous granules and a pericentric inversion of chromosome 16. Surface marker analysis of leukemic cells from these patients, using a panel of monoclonal antibodies, revealed the expression of a series of monocyte markers. The association of an inversion of chromosome 16 with abnormal eosinophil morphology in the M4 subtype of ANLL appears to represent a unique subgroup of patients.