Abstract BACKGROUND Larotrectinib is the first-in-class, highly selective TRK inhibitor approved for tumor-agnostic use in patients with TRK fusion cancer. We report data on larotrectinib-treated pediatric patients with TRK fusion primary CNS tumors. METHODS Patients aged <18 years with TRK fusion primary CNS tumors enrolled in two clinical trials (NCT02637687, NCT02576431) were included. Responses were independent review committee (IRC)-assessed per RANO. Patients were permitted to stop larotrectinib in the absence of on-treatment progression (wait-and-see) and remain on trial. RESULTS As of July 2023, 38 patients were eligible for efficacy analyses by IRC, including 11 patients with only non-measurable disease at baseline. Median age at enrollment was 7 years (range 0–17). Tumor histologic groups included: high-grade glioma (HGG; n=18), low-grade glioma (LGG; n=12), and other (n=8). Eleven (29%) patients had received ≥2 prior systemic therapies. Overall response rate was 37% (95% CI 22–54): three complete response, 11 partial response, 17 stable disease, five progressive disease, and two not evaluable. The 24-week disease control rate was 74% (95% CI 57–87). Medians for duration of response, progression-free survival, and overall survival (OS) were 17.2 months (95% CI 5.7–not estimable NE), 19.8 months (95% CI 11.1–50.8), and not reached (95% CI 32.7–NE), respectively. The 48-month OS rate was 60%. At data cutoff, one patient with HGG and four with LGG entered wait-and-see; median duration of the first wait-and-see period was 20.2 months (range 4–29). One patient with LGG resumed treatment following wait-and-see. Treatment-related adverse events (TRAEs) were predominantly Grade 1/2. No patients discontinued treatment due to TRAEs. CONCLUSION Larotrectinib demonstrated rapid, durable responses and a manageable safety profile in pediatric patients with TRK fusion primary CNS tumors. This supports the wider adoption of next-generation sequencing panels that include NTRK gene fusions when testing pediatric patients with CNS tumors.