The goals of this study were to validate a number of available collective social capital measures at the US state and county levels, and to examine the relative extent to which these social capital measures are associated with population health outcomes. Measures of social capital at the US state level included aggregate indices based on the Annenberg National Health Communication Survey and the Behavioral Risk Factor Surveillance System (BRFSS), Petris Social Capital Index (PSCI), Putnam's index, and Kim et al.'s scales. County-level measures consisted of PSCI, Rupasingha et al.'s social capital index, and a BRFSS-derived measure. These measures, except for the PSCI, showed evidence of acceptable validity. Moreover, we observed differences across the social capital measures in their associations with population health outcomes. The implications of the findings for future research in this area were discussed.