OBJECTIVE: To supply information on the efficacy of on-site day treatment for homeless mentally ill men in shelters, the authors followed up homeless mentally ill men 18 months after placement in community housing. METHOD: The 42 subjects had been evaluated before and 6 months after entering an on-site day treatment program. The authors reinterviewed 34 of these patients again 1 year after the first follow-up to determine housing status, hospitalization, aftercare, criminal justice contacts, income, and employment. RESULTS: By the 18- month follow-up the positive effects of the program at 6 months had deteriorated; 44% of the men had returned to shelters at some point during the follow-up period, and the number of men with criminal justice contacts had increased to a proportion exceeding that before the program. A concurrent diagnosis of substance abuse increased the risk of homelessness during follow-up. CONCLUSIONS: These findings underscore the need for innovative treatment and support services for the homeless mentally ill who have concurrent substance abuse.Abstract Teaser