OBJECTIVE: Clients' and providers' perceptions of clients' needs were compared in 18 community treatment programs participating in the Access to Community Care and Effective Services and Supports program of the Center for Mental Health Services, a national demonstration project on treatment of homeless persons with mental illness. The study sought to determine whether perceptions differed and whether assessed needs for services were related to service use. METHODS: A total of 1,482 clients contacted through community outreach who entered the case management phase of the program after an average of 32 days were given an evaluation interview at entry into the program. The clients and outreach workers identified clients' needs in seven core domains-mental health, general health, substance abuse, public financial support, housing assistance and support, dental care, and employment. Use of related services in the 60 days before the case management evaluation was determined. RESULTS: The greatest differences between clients' and providers' perceptions of service needs were in dental and medical services, which were more frequently identified as needs by clients, and in substance abuse and mental health services, which were more frequently identified by providers. Clients' and providers' assessments of need were significantly, but not strongly, correlated with each other, and both were correlated with use of mental health and substance abuse services. CONCLUSIONS: Mental health service providers are less likely than clients to identify needs for services other than mental health services. Service use, at least in the short run, is related to both clients' and providers' assessments of need.