Two patients with acute nonlymphocytic leukemia (ANLL) and t(16;21)(p11;q22) were studied. The patients exhibited such clinical and hematological pictures, characterized by M2 and M4 with eosinophilia (FAB classification), as relatively matured leukemic cells, low neutrophil alkaline phosphatase activity, abnormal eosinophils and a high count of monocytic cells in the bone marrow. The prognosis was poor in both patients. From these data, the chromosomal abnormality of t(16;21)(p11;q22) seems to be specifically associated with a unique subtype of ANLL.