Discrepancies between informants’ reports of children’s behavior are robustly observed in clinical child research and have important implications for interpreting the outcomes of controlled treatment trials. However, little is known about the basic psychometric properties of these discrepancies. This study examined the relation between parent-child reporting discrepancies on measures of child social phobia symptoms, administered before and after treatment for social phobia. Participants included a clinic sample of 81 children (7–16 years old [M = 11.75, SD = 2.57]; 39 girls, 42 boys) and their parents receiving treatment as part of a multisite controlled trial. Pretreatment parent-child reporting discrepancies predicted parent-child discrepancies at posttreatment, and these relations were not better accounted for by the severity of the child’s pretreatment primary diagnosis. Further, treatment responder status moderated this relation: Significant relations were identified for treatment non-responders and not for treatment responders. Overall, findings suggest that informant discrepancies can be reliably employed to measure individual differences over the course of controlled treatment trials. These data provide additional empirical support for recent work suggesting that informant discrepancies can meaningfully inform understanding of treatment response as well as variability in treatment outcomes.