There is very little known about health care utilization among the homeless or about the role of health insurance on utilization patterns. Many health care reform proposals advocate expanding health insurance coverage for various segments of society, including the homeless. Although homeless people who lack health insurance face strong financial barriers to health services, providing them with health insurance may not appreciably increase their demand for health care if they also face important non-financial barriers. We investigate the relationship between insurance and utilization for this group based on estimates from an empirical model of medical care use and insurance coverage. Using our estimates, we simulate potential effects of policy changes on various types of utilization, including use of mental health services and treatment for alcohol or other drug abuse.