This report presents the methodology and outcomes of an evaluation of the Chemical Dependency Recovery Program (CDRP) of Southern California Kaiser-Permanente, a large group model HMO. CDRP treatment includes specialized detoxification inpatient facilities, day treatment and outpatient services. Substance use outcomes in a population of 1,986 patients consecutively entering treatment were obtained via telephone at six and twelve months post intake. For patients who consumed only alcohol at baseline (N = 621), significantly higher six month point prevalence abstinence rates were found among those who remained in treatment for a minimum of three months (89%), compared to those patients who failed to return for treatment after their intake (31%). For polydrug users (N = 1365), as contrasted with patients who utilized services only on their intake date, significantly higher six month point prevalence rates were restricted to those who remained in treatment for a minimum of six months (74% versus 21%). The evaluation documented the need for CDRP services, validated program effectiveness and highlighted areas for systematic improvement.