Nineteen patients with acute nonlymphocytic leukemia were found to have a pericentric inversion of chromosome 16. All were classified as having acute myelomonocytic leukemia (M4) according to French-American-British Cooperative Group (FAB) criteria. Consistent abnormalities of marrow eosinophils were present. In 15 of the 19 patients, the number of eosinophils was increased. Morphologic abnormalities of eosinophils were present in all patients, the most striking being the presence of large basophilic staining granules in 18 of the 19 patients. In most cases we also found a population of cells with morphologic features of both eosinophils and monocytes. Eosinophils showed atypical cytochemical reactions with PAS in ten of ten cases studied, with naphthol ASD Chloroacetate in ten of ten cases, and with combined naphthol ASD Chloroacetate esterase- chlorazol fast pink in seven of nine cases tested. The presence of inv(16) defines a distinct subset of ANLL with consistent morphologic and cytochemical features.