The results indicated that youth reported more substance use than their parents perceived. Results indicated moderate agreement between urinalysis and youth self-report. The subjective youth reports of substance use showed greater agreement with urinalysis results than did any of the parental assessments. However, while there was no significant difference between urinalysis and parent report at the 3-month follow-up, there was a significant difference at the 9-month follow-up. The agreement between parent and youth reports was similar for the 3- and 9-month follow-ups. At both the 3- and 9-month follow-ups, parents’ perceptions of their child’s substance use significantly predicted the change in youth’s status so that the higher the parent’s view of the youth, the more progress the youth perceived themselves to have made. The findings have implications for research studies using youth- and parent-report questionnaires regarding substance use. Participants were 88 adolescents who had been referred to an out-patient drug treatment program during the period 1996 through 1998. Participants completed the Teen Addiction Severity Index and provided a sample for urinalysis, which tested for cannabinoid, cocaine, and opiates. Participants were then randomly selected into 1 of 2 treatment conditions: CBT (n=51) or PET (n=37). Following the 8-week treatment condition, adolescents were assessed at 3 months following program completion and then again at 9 months following program completion. Parents of the adolescent participants completed Parent Collateral-Report at each of the three data collection points (baseline, 3 months after program completion, and 9 months after program completion). Data analysis involved the use of McNemar’s test and Pearson correlations. Tables, references