Objective: It has been proposed that social integration would act as a confounder in the relationship between alcohol consumption and all-cause mortality. This study tested the assumption that the J-shaped relationship between drinking and all-cause mortality may partly reflect a protective effect of social integration, to the extent that moderate drinkers are more socially integrated than either abstainers or heavy drinkers, and to the extent that social integration offers direct protection from mortality. Method: This hypothesis was tested using data from 10,832 of the 12,866 men in the Multiple Risk Factor Intervention Trial (MRFIT). Indicators of social integration were derived from an exploratory factor analysis of 25 relevant items in the MRFIT data and from a scale of six items selected by the investigators. Results: We failed to confirm a direct protective effect of social integration. Nondrinkers had the highest rates of all-cause mortality. Compared with heavy drinking, relative risks of all-cause mortality for abstinence, light and moderate drinking were unaffected by inclusion of social integration variables in the proportional hazards models. Conclusions: The MRFIT data fail to confirm a confounding effect of social integration.