OBJECTIVE: The prevalence of mental illness and substance abuse in homeless populations has been studied primarily in large urban areas. This study examines a sheltered homeless population in two counties of lower-density population, Dauphin and Cumberland counties in central Pennsylvania, to assess the prevalence of mental illness and substance abuse. METHODS: A total of 81 homeless adults from nine emergency shelters were interviewed using a structured questionnaire. RESULTS: The estimated lifetime prevalence rate of major depressive disorder was 26.6 percent; 6.4 percent of the sample showed evidence of psychotic thinking. Almost one-third reported previous hospitalization for emotional problems, and about one-third reported a suicide attempt. The estimated lifetime prevalence rate of alcohol or drug abuse or dependence was almost 60 percent. CONCLUSIONS: Although mental illness, especially psychosis, and substance abuse may be somewhat less prevalent among homeless persons in lower-density population areas than in large urban areas, they are nevertheless significant problems.