This article describes the development and psychometric assessment of the Evaluation of Client Services (ECS) measure of treatment satisfaction for people with chronic mental illnesses in community treatment settings. The ECS, a 20-item instrument, was validated in a sample of 184 individuals receiving outpatient mental health treatment in New York City. The four dimensions of the ECS, (1) treatment management and treatment outcome, (2) treatment relationship, (3) communication and information exchange, and (4) reachability of treatment facilities are internally consistent and stable over time. Analyses also show that the ECS is a valid indicator of satisfaction with mental health services which meaningfully correlates with quality of life and another measure of treatment satisfaction. The ECS is a brief and easy to understand treatment satisfaction tool with good psychometric properties.