Social relationships serve important functions in people's everyday lives. Epidemiological research indicates that supportive relationships may also significantly protect individuals from various causes of mortality, including cardiovascular disease. An important issue is how social support influences such long-term health outcomes. In this article, we review evidence indicating that social support may influence mortality via changes in the cardiovascular, endocrine, and immune systems. These data suggest that it may be worthwhile to incorporate social-support interventions in the prevention and treatment of physical health problems.