Background: AML is characterized by maturation arrest of a malignant clone of myeloid cells. Signal transducer and activator of transcription 5 (STAT5) is a transcription factor that regulates many as¬pects of cell growth, survival and differentiation. Constitutive activation of STAT5 has been identified in a number of hematopoietic malignancies including AML.Aim: The aim of this study was to assess the protein expression of phosphorylated-STAT5 in leukemic blasts of newly diagnosed AML patients to correlate its expression with the clinical outcome in these patients. Subjects and Methods: Thirty patients suffering from de novo AML were included in the study. Assessment of phosphorylated-STAT5 protein in AML blasts of bone marrow aspirate/peripheral blood smear films was performed by immunocytochemistry using rabbit anti-phospho-STAT5A/B (Y694/Y699) and fluorescence-labelled anti-rabbit IgG antibodies.Results: Expression of total p-STAT5 was detected in 23/30 (76.7%) of cases. Expression of cytoplasmic and nuclear p-STAT5 was detected in 24/30 (80%) and in 7/30 (23.3%) of cases, respectively. Median blast count percentage in peripheral blood and bone marrow at presentation was significantly higher in the total p-STAT5 positive AML patients than the p-STAT5 negative AML patients (median 30% vs. 10%, P=0.028 and median 60% vs. 45%, P=0.048; respectively). No statistically significant difference was found between the response to induction therapy and p-STAT5 expression. Conclusion: The present study confirmed the expression of total phosphorylated-STAT5 in about two-thirds of newly diagnosed AML cases. Cytoplasmic p-STAT5 was detected in a larger set of AML patients than nuclear p-STAT5. No statistically significant impact of p-STAT5 expression was encountered on the clinical outcome in these patients.