Self-reports continue to be the most commonly used method in studies on the effectiveness of treatments for drug addiction. Nevertheless, the validity of this procedure has been called into question, so that subjects' reports are often compared with data from other sources. This study compares the data collected from the self-reports of 207 subjects that had participated in a drug-free treatment program with information from the family and with official records from the main assistance units for drug addicts in the region (Asturias, Spain). As regards misuse of illegal drugs and alcohol, the results show a high level of agreement (91.3% and 90.35%, respectively) between self-reports and information from the family. The data are similar for the other variables studied (work and educational activity, leisure activities, and family relationships). Agreement between self-reports and data collected from the assistance units is very high, reaching 91.19%. These results support the hypothesis that self-reports may constitute a valid instrument for assessing the results of drug-addiction treatment programs.