Eosinophilic leukemias are difficult to individualize amid the hypereosinophilic syndromes. Chromosomal abnormalities when present within the eosinophils are of critical value in the diagnosis of a malignancy. We report here the case of a 27-year-old woman who had been healthy, until recently when she suddenly developed hepatosplenomegaly and lymph node enlargement, and considerable eosinophilia in blood and bone marrow. The morphologically abnormal cells (large pseudo Pelger eosinocytes) predominated in the cytology. The establishment in these cells of a clonal chromosomal anomaly, t(10;11)(p14;q21), favored the malignancy and diagnosis of acute eosinophilic leukemia.