We conducted an urban “metabolic” study of inflows and outflows of food, water, energy, and pollutants from Los Angeles County, USA. This region has been the subject of recent debate about the nature of population density and distribution as it relates to urban form and associated environmental impacts. We found that with the exception of food imports, inputs of resources and outputs of pollutants generally declined on a per capita basis from 1990 to 2000. Reductions likely reflected a combination of changes in public policy, investment in public infrastructure, and impacts of increases in population density. However, in comparison to other municipalities and urban regions, resource consumption per capita was still relatively high in some categories, particularly water imports and consumption of transportation fuel. Per capita greenhouse gas emissions were lower than in the United States as a whole but higher than previous analyses of urban areas, largely due to comparatively high emissions from the transportation sector. While there is a large literature on the environmental impacts of vehicle emissions in Los Angeles, there is comparatively little information about the nature and impacts of water losses, such as the magnitude and spatial variability of evapotranspiration and its effect on urban energy balance in this region. We suggest that more detailed studies of water losses will greatly improve future mass and energy accounting.