While excellent adolescent alcohol and drug screening tools are available, there are relatively few, if any, psychometrically validated measures to use in the assessment of adolescent treatment outcome. This study conducted a test-retest exercise of the Form 90 Drug and Alcohol (Form 90 DnA) to determine the stability of adolescent responses when administering the day-by-day calendar/grid approach. Homeless youth (N = 37) with alcohol, drug, or alcohol and drug abuse/dependence combined were recruited to participate in the test-retest study. High pre-post stability in means was obtained on measures of frequency of substance use in general, and on specific measures of alcohol, cocaine, marijuana use. The findings from this paper provide support for the reliability and validity of the Form 90 for use with adolescent runaways with a substance abuse or dependence diagnosis.