The authors prospectively examined the effects of social ties and change in social ties, as measured by a well-known social network index, on total and cause-specific mortality and on coronary heart disease incidence in 28,369 US male health professionals aged 42-77 years in 1988. Over 10 years, the relative risk of total mortality for men in the lower two levels of social integration compared with more socially integrated men was 1.19 (95% confidence interval: 1.06, 1.34) after controlling for age, occupation, health behaviors, general physical condition, coronary risk factors, and dietary habits. In multivariate analysis, deaths from accidents and suicide and from other noncancer, noncardiovascular causes were significantly increased among less socially connected men. Socially isolated men also had an increased risk of fatal coronary heart disease (multivariate relative risk = 1.82, 95% confidence interval: 1.02, 3.23). An increase in the overall social network index between 1988 and 1996 was not significantly associated with subsequent 2-year mortality. In analyses of change in social network components restricted to older men, each categorical unit increase in number of close friends was significantly associated with a 29% decrease in risk of death. Increase in religious service attendance over time was also significantly predictive of decreased mortality.