This is a prospective cohort study of 150 individuals attending a specialized health service for substance-related disorders. The study investigated the association between motivation to remain in treatment and treatment adherence. All service users were interviewed soon after admission to the treatment program and were followed-up during the first two months of treatment. A Cox Regression Model was used to estimate the hazard ratios for dropout during the two months following the admission interview. The results indicated that individuals with a primary-school education, lack of income, and low motivation toward treatment at the admission interview presented a higher risk of treatment dropout. This study showed the importance of motivation in changing addictive behavior and in adherence to treatment as essential factors for recovery.