The role of suppressors of cytokine signaling (SOCS)-3 and -1 has been investigated in various cancers. These proteins have been identified as endogenous controllers of activation of Janus kinase/signal transducers and activators of transcription factors pathway factors under physiological conditions and in disease. SOCS-3 expression is lost in several cancers due to epigenetic mechanisms, mostly promoter methylation. In liver, lung, and squamous head and neck cancer, and several hematological malignancies SOCS-3 acts as a classic tumor suppressor. In prostate cancer, SOCS-3 effects are cell type-dependent. It prevents apoptosis in androgen receptor- negative cells. However, in androgen-sensitive cells, it could act as a negative feedback factor for androgenic regulation. Melanoma cells which overexpress SOCS-3 confer a growth advantage. SOCS-1 is in most cancers a tumor suppressor which may inhibit expression of cyclin- dependent kinases and cyclins. In general, the mechanisms responsible for the different effects of SOCS in cancer cell lines have to be further investigated. The results discussed in the present review may have an impact on personalized approaches in cancer medicine.