Microbiota and innate immune mechanisms regulate a proinflammatory microenvironment promoting tumor growth. Possibly as the result of a defective intestinal barrier, microbial products activate innate immune cells or intestinal epithelial cells (IECs), perhaps via TLRs, which then create a proinflammatory microenvironment with subsequent activation of Th17 cells characterized by the transcription factor RAR-related orphan receptor γT (RORγt). Proinflammatory cytokines released by activated innate immune cells, intestinal epithelial cells, or Th17 cells contribute to tumor cell proliferation and survival.