OBJECTIVE: Three types of case management were compared to determine their relative effectiveness in helping people with severe mental illness who were homeless or at risk of homelessness. METHODS: Subjects recruited from a psychiatric emergency room and inpatient units were randomly assigned to one of the three treatment conditions: broker case management, in which the client's needs were assessed, services were purchased from multiple providers, and the client was monitored; assertive community treatment only, in which comprehensive services were provided for an unlimited period; and assertive community treatment augmented by support from community workers, who assisted with activities of daily living and were available for leisure activities. Of 165 subjects recruited, 135 were followed for 18 months. RESULTS: Compared with clients assigned to broker case management, clients assigned to assertive community treatment only and assertive community treatment with community workers had superior outcomes on several variables. They were number of contacts with the assigned treatment program, resource utilization (for example, use of entitlements), severity of thought disorder, activity level, and client satisfaction. Clients in the assertive community treatment only condition achieved more days in stable housing than those in the other two treatment conditions. No significant treatment group effects were found on income, self-esteem, or substance abuse. CONCLUSIONS: Assertive community treatment is superior to broker case management in assisting individuals with serious mental illness who are at risk of homelessness.