The relationship between ethnic density (the relative size of a given ethnic group in a multi-ethnic neighborhood) and risk for psychiatric hospitalization was explored for black, white, and Puerto Rican residents of New York City's 338 health areas. The author found that the smaller the ethnic group, the higher its hospitalization rate in comparison to both the rate of other residents in the same area and that of members of the same ethnic group living in areas where they constituted a numerical majority. Analysis of sociodemographic variables showed that the effect of ethnic density on risk for psychiatric hospitalization cannot be accounted for by differences in poverty, family cohesiveness, or population mobility.