A study of homeless people in Baltimore, Md, focused on their health and other characteristics, with special emphasis on their needs for services. In the first stage, 298 men and 230 women were randomly selected from the missions, shelters, and jail in Baltimore to respond to a baseline interview that provided extensive sociodemographic and health-related data. In the second stage, a subsample of 203 subjects was randomly selected from the baseline survey respondents to have systematic psychiatric and physical examinations. Data are presented from both stages. Data from the first stage demonstrate, among other things, the high levels of disaffiliation of this population and their heavy involvement in substance abuse. Data from the clinical examinations demonstrate the high prevalence of mental illnesses and other psychiatric disorders and of a wide range of physical disorders and confirm the high prevalence of alcohol abuse disorders. The high rates of comorbidity of these conditions is demonstrated and data are provided on the subjects' needs for mental health and substance abuse services.