OBJECTIVES: To review evidence that the immune system plays a role in controlling the spread of cancer and findings that perioperative pain relief improves immune status and health outcomes. DATA SOURCES: Research studies and review articles pertaining to immunity, immune function, stress, and immune-suppressive nature of pain. CONCLUSIONS: Pain not only results in suffering but is a pathogen itself, capable of facilitating the progression of metastatic disease. Adequate pain relief decreases these risks. IMPLICATIONS FOR NURSING PRACTICE: Adequate pain relief is not only a primary concern in caring for individuals in pain but may be a matter of physiologic necessity as further studies reveal the immune-suppressive nature of pain.