Clinical data were gathered on 627 homeless Vietnam veterans evaluated in a Department of Veterans Affairs clinical program for homeless mentally ill veterans. More than two-fifths (43 percent) of the 627 veterans showed evidence of combat stress that was associated with more severe psychiatric and substance abuse problems, although not with greater social dysfunction. In comparison with Vietnam veterans assessed in a national epidemiological study, homeless veterans were severely socially and vocationally dysfunctional. While homeless mentally ill veterans with combat stress used VA mental health services more frequently than did homeless mentally ill Vietnam veterans with other disorders, many received no mental health services. Combat stress appears to be a significant problem among homeless mentally ill Vietnam veterans.