Introduction: Treatment trials under development for Focal Cerebral Arteriopathy (FCA)—a unilateral anterior circulation arteriopathy that causes childhood arterial ischemic stroke—require a validated metric for FCA severity. The FCA Severity Score (FCASS) quantifies disease severity and extent. In the derivation cohort, FCASS peaked at 1-4 weeks post-stroke, and maximum FCASS correlated with infarct size and one-year Pediatric Stroke Outcome Measures (PSOM). The goal of the current study was to validate this score in an independent cohort. Methods: We used banked neuroimaging and previously acquired 6-month PSOM scores for 32 children with FCA in the Swiss Neuro-Paediatric Stroke Registry (SNPSR). A pediatric neuroradiologist applied FCASS to all available vascular imaging. FCASS sums the individual severity scores (from 0 for no involvement, to 4 for occlusion) for 5 arterial segments (supraclinoid ICA, M1, M2, A1, A2). The same neuroradiologist measured Pediatric ASPECTS scores, a measure of infarct volume (higher scores representing greater volumes, with a maximum possible score of 30). Results: Of 32 cases, 6 had only acute vascular imaging (≤1 day post-stroke), and 2 had only delayed vascular imaging (>3 months post-stroke). Including all 32 cases, the maximum FCASS score at any time was a median of 9 (range 3, 15; IQR 6, 12). Maximum FCASS correlated with Pediatric ASPECTS (Spearman’s rho=0.616, p=0.0002; Figure). The median PSOM was 0.5 (range 0 [no deficit], 3 [moderate deficit]; IQR 0, 1.5). Although Pediatric ASPECTS correlated with PSOM (Spearman’s rho=0.623, p=0.0002), there was no direct correlation between maximum FCASS and PSOM (rho=0.25, p=0.185). Conclusion: Limitations include a small sample size and missing imaging during the peak FCASS window for 8 of 32 cases. Regardless, in this independent validation cohort, higher maximum FCASS correlated with greater infarct volume scores that also correlated with worse neurological outcomes.