The human proto-oncogene STAT3 encodes transcriptional factor that is essential for embryo- genesis, proliferation and differentiation of many cell types, in addition to organ regeneration and involu- tion. It also regulates innate and adaptive immune mechanisms, including B cell maturation. Augmented STAT3 activation has been found in both irreversibly committed B cell lineage precursors and plasmacy- tes. Abnormal STAT3-mediated signal transduction has been observed in a large number of neoplasms, in which it induces anti-apoptotic and cell cycle progression mediating genes transcription. In many tumor types, also of lymphoid origin, STAT3 facilitates tumor expansion due to angiogenesis stimulation and attenuation of anti-cancer immunity. STAT3 can be considered as a perfect therapeutic target.