The Form 90 combines the strengths of the daily calendar approach to reconstruct substance use (Sobell and Sobell, 1992) with a weekly-grid procedure in order to expedite the interview process. Form 90 has already demonstrated excellent test-retest reliability with adult alcohol abusers seeking treatment. For the current study, runaway youth (n=37) were recruited through two runaway/homeless shelters in a southwestern U.S. city as part of two larger studies that were examining family therapy outcome with substance-abusing runaway adolescents. All of the youth in the sample agreed to the possibility of family treatment with a parent or surrogate parent. Eligibility for participation required that each adolescent satisfy DSM-IV diagnostic criteria for substance abuse or dependence, be between the ages of 12 to 17, and live within a 60-mile radius of the research site. By design, this test-retest study recruited a sample that would provide a conservative estimate of the reliability of the Form 90 when administered to adolescents. Forty-six percent of the youth met criteria for alcohol and drug abuse/dependence combined. Equally important, homeless youth in the sample had substantially less social structure and stability than typical substance-using adolescents. Thus, the reconstruction of daily substance use and health care activities was especially challenging for this group compared to most adolescents. Still, most measures collected using the Form 90 were reliable in this test-retest exercise. Adolescents were reasonably consistent in reporting the use of global and specific substances through the daily calendar approach. Compared with global drug use, however, less consistency was obtained among measures of specific drug use. On one hand, adolescents in the sample were reasonably consistent in reporting the frequency of alcohol and cocaine use for the prior 90 days. Less agreement was obtained for the frequency of marijuana use. Thus, Form 90 may not be a reliable measure for marijuana use, at least for homeless adolescents. Although additional psychometric work with a larger sample is required, this study's findings constitute an important first step in documenting the reliability of the Form 90 with adolescents. 4 tables and 27 references