Glioblastoma multiforme (GBM) modulates the immune system to engance its malignant potential. Signal transducer and activator of transcription 3 (STAT3) activation is a regulatory node in modulating the immune microenvironment in several human tumors, including GBM. To investigate whether STAT3 inhibition might enhance anti-tumor responses, we inhibited STAT3 signaling using small interfering RNA against STAT3. We tested the human GBM cell lines U87, U251, and HS683, which are known to constitutively express high levels of phospho-STAT3. STAT3 inhibition resulted in enhanced expression of several pro-inflammatory cytokines and chemokines and supernatants from STAT3-silenced human GBM cell lines increased lipopolysaccharide-induced dendritic cell activation in vitro. We obtained comparable results when STAT3 activity was suppressed with specific small molecule inhibitors. Our results support the hypothesis that activated STAT3 contributes to the immunosuppressive microenvironment in GBM and support previous studies implicating STAT3 as a potential target for immunotherapy.