Substance abuse and mental disorders increase the health care needs of homeless persons, whose primary source of care is often the emergency room. In this study, associations between substance abuse and mental health problems and use of emergency rooms were examined using data from a 1987 survey of 1,152 homeless adults in New York City shelters. Two-thirds (N=767) of the sample reported using an emergency room at least once in their lives, and 27.2 percent (N=313) reported use within the previous six months. Traumatic injury was the most frequently cited reason for the last emergency room visit. Respondents who reported psychotic ideation during the previous year or severe depressive symptoms during the previous week were much more likely to have used an emergency room within the previous six months. The preliminary findings suggest that substance abuse and mental disorders play a significant role in emergency room use by homeless adults.