A series of studies have demonstrated that people who live in regions where there are disparities in income have poorer average health status than people who live in more economically homogeneous regions. To test whether such disparities might explain health variations within urban areas, we examined the possible association between income inequality and infant mortality for zip code regions within New York City using data from the 1990 census and the New York City Department of Health. Both infant mortality and income inequality (percentage of income received by the poorest 50% of households) varied widely across these regions (range in infant mortality: 0.6-29.6/1,000 live births; range in income inequality: 12.7-27.3). An increase of one standard deviation in income inequality was associated with an increase of 0.80 deaths/1,000 live births (P <.001), controlling for other socioeconomic factors. This finding has important implications for public health practice and social epidemiological research in large urban areas, which face significant disparities both in health and in social and economic conditions.