Studies have shown that poverty and income are powerful predictors of homicide and violent crime. We hypothesized that the effect of the growing gap between the rich and poor is mediated through an undermining of social cohesion, or social capital, and that decreased social capital is in turn associated with increased firearm homicide and violent crime. Social capital was measured by the weighted responses to two items from the U.S. General Social Survey: the per capita density of membership in voluntary groups in each state; and the level of social trust, as gauged by the proportion of residents in each state who believed that "most people would take advantage of you if they got the chance". Age-standardized firearm homicide rates for the years 1987-1991 and firearm robbery and assault incidence rates for years 1991-1994 were obtained for each of the 50 U.S. states. Income inequality was strongly correlated with firearm violent crime (firearm homicide, r = 0.76) as well as the measures of social capital: per capita group membership (r = -0.40) and lack of social trust (r = 0.73). In turn, both social trust (firearm homicide, r = 0.83) and group membership (firearm homicide, r = -0.49) were associated with firearm violent crime. These relationships held when controlling for poverty and a proxy variable for access to firearms. The profound effects of income inequality and social capital, when controlling for other factors such as poverty and firearm availability, on firearm violent crime indicate that policies that address these broader, macro-social forces warrant serious consideration.