Signal transducer and activator of transcription 5 (STAT5) activity is induced by a plethora of cytokines and growth factors resulting in transcriptional activation of specific target genes. STAT5 plays an important role in a variety of cellular processes, including proliferation, differentiation, and apoptosis. Aberrant regulation of STAT5 has been observed in solid tumors as well as in patients with either chronic or acute myeloid leukemia. Kinase inhibitors are currently being developed to negatively regulate STAT5 activity for clinical purposes.