Objective: To assess the mental health of homeless and poor housed youths, using the National Institute of Mental Health (NIMH) Diagnostic Interview Schedule for Children (DISC) Version 2.3, and to examine mental health service use. Method: As part of a comprehensive study of homeless and housed families in Worcester, MA, data were collected on 41 homeless and 53 poor housed (never homeless) youths aged 9 to 17 using both the parent and youth versions of the DISC. Results: On the basis of the parent version of the DISC, current (6-month) prevalence rates of DSM-III-R disruptive behavior, affective, and anxiety disorders were comparable in homeless and housed youths but higher than rates found among youths in the NIMH-sponsored Methods for the Epidemiology of Child and Adolescent Mental Disorders (MECA) Study, which used the same diagnostic measure. Approximately 32% of the combined sample of homeless and housed youths had a current mental disorder accompanied by impairment in functioning. Mental health service use in the preceding 6 months among youths who had one or more current disorders and associated impairment ranged from 20% to 35%. A subgroup of youths with one or more current disorders and poor global functioning had never received treatment. Conclusions: This sample of homeless and housed youths was found to have high rates of current mental disorders. Use of mental health services by children with mental health needs was low, particularly for youths with poor overall functioning. (Authors)