Interleukin-6, levels of which are elevated in prostate cancer, activates different signal transduction pathways including that of Janus kinases/signal transducer and activator of transcription (STAT)3. However, phosphorylation of STAT3 has been reported to be associated with either stimulatory or inhibitory effects on cellular proliferation. To better understand the mechanisms of STAT3 regulation in benign and malignant prostate, we have investigated the role of suppressor of cytokine signaling (SOCS)-3. Cell lines that did not express phosphorylated STAT3 were found to be SOCS-3-positive. SOCS-3 was re-expressed in LNCaP cells after treatment with a demethylating agent. SOCS-3 immunohistochemistry revealed a negative or weak reaction in benign areas, whereas its expression was detected in tumor tissue. To investigate the involvement of SOCS-3 in regulation of cellular events, we incubated cancer cells with a cAMP derivative. This treatment yielded higher SOCS-3 levels, reduced [3H]thymidine incorporation, and increased percentage of apoptotic cells. However, down-regulation of SOCS-3 by a short interfering RNA approach resulted in inhibition of proliferation and an increased apoptotic rate. Collectively, our results show that SOCS-3 antagonizes regulation of cellular events by cAMP and is expressed in human prostate cancer.