This study compared demographic and diagnostic characteristics of a sample of homeless outpatient mental health clinic attenders with a domiciled comparison group from the same clinic. Data on demographic variables and DSM-III-R psychiatric diagnoses were collected over a two-year period on a consecutive sample of 166 homeless and 117 nonhomeless clinic attenders. Data on demographics and psychiatric diagnoses of the homeless clinic attenders were further compared with data collected systematically from 900 homeless individuals in the same city. In the clinic, homeless subjects were more often members of ethnic minorities, and homeless women were significantly younger and better educated than their nonhomeless counterparts. Rates of schizophrenia, bipolar disorder, and somatization disorder were not significantly different between homeless and nonhomeless groups. Major depression was about four times as prevalent in nonhomeless men as in homeless men. Homeless men were significantly more likely than nonhomeless men to qualify for a diagnosis of alcohol use disorder, and homeless women were more likely than other women to qualify for a diagnosis of drug use disorder. Both homeless men and women were significantly more likely than their domiciled counterparts to meet criteria for antisocial personality disorder. Personality disorder other than antisocial was more prevalent in nonhomeless men than in homeless men. Combined rates of personality disorder were significantly higher among homeless than nonhomeless women, but not men. Homeless clinic attenders were demographically and diagnostically very similar to a general homeless population in the same city. The only diagnosis that was more prevalent in the homeless clinic than in the homeless community was antisocial personality disorder. We concluded that because of difference in diagnostic prevalence, homeless and nonhomeless individuals in mental health clinics need to be considered independently. Clinicians treating homeless outpatients may benefit from having special facility in diagnosis and management of antisocial personality disorder and substance abuse, along with expertise in other psychiatric disorders in this population.