Updated project metadata. The pathogenesis of multiple sclerosis (MS) remains to be elucidated. Pediatric-onset MS (POMS) represents the earliest stage of the disease. CSF proteins in POMS may therefore provide causal information. Therefore, the aim of the current study was to analyze CSF proteins in children with an initial CNS acquired demyelinating syndrome (ADS) and to make a comparison between POMS and monophasic ADS (mADS). Patients were selected from two prospective pediatric ADS studies. Liquid chromatography-mass spectrometry was performed in a Dutch discovery cohort (POMS n=28; mADS n=39). Parallel reaction monitoring-mass spectrometry was performed on selected proteins more abundant in POMS with ≥ 8 unique peptides in a combined Dutch and Canadian validation cohort (POMS n=48; mADS n=106).