Chronic eosinophilic leukemia-not otherwise specified (CEL-NOS) is a rare disorder with hypereosinophilia and an increased number of blood or marrow blast (<20%) or an evidence of eosinophil clonality.We evaluated the clinical outcome of 10 patients with CEL-NOS. Seven males and three females at a median age of 62 years (range, 23–73) were included. The median leukocyte count at diagnosis was 33.4 3109/l (range, 9.3–175.0) with a median eosinophil count of 15.6 3 109/l (range, 1.5–136.0). Median hemoglobin and platelets were 11.0 g/dl (range, 8.3–13.3) and 158 3 109/l (range, 31.0–891.0), respectively. Clinical manifestations included splenomegaly (n 5 7), hepatomegaly (n 56), cardiac failure (n 5 2), and lung infiltrations (n 5 1). Median survival from diagnosis to death for entire cohort was 22.2 months (range,2.2–186.2). Five of the 10 studied patients developed acute transformation(AT) after median of 20 months from diagnosis (range, 1.6–41.9).None of patients with AT is alive at the time of last follow-up. Median time from AT to death was 2 months (range, 1.0–6.1). Among five patients who did not develop AT, three died in active disease. Two patients are alive in complete remission; first underwent allogeneic stem-cell transplantation preceding by intensive induction chemotherapy;the second remains on imatinib with hydroxyurea. Except the latter patient, imatinib was ineffective in our study population. CEL-NOS is a rare and aggressive disease with high rate of AT and resistance to conventional treatment.