The Pediatric Symptom Checklist (PSC) is a widely-used, parent-completed measure of children’s emotional and behavioral functioning. Previous research has shown that the PSC and its subscales are generally responsive to patient progress over the course of psychiatric treatment. In this naturalistic study, we examined the performance and utility of the five-item PSC Internalizing Subscale (PSC-IS) as an assessment of routine treatment in outpatient pediatric psychiatry. Parents and clinicians of 1,593 patients aged 17 or younger completed standardized measures at intake and three-month follow-up appointments. Comparisons between PSC-IS scores and clinician-reported diagnoses, internalizing symptoms, and overall functioning showed acceptable levels of agreement. Change scores on the PSC-IS were also larger among patients with internalizing diagnoses than those with non-internalizing diagnoses. As a brief measure of internalizing symptoms, the PSC may be particularly useful to mental health clinicians treating youth with depression and anxiety as a quality assurance or treatment outcome measure.