['The clinical and MRI presentation differs between earlier- and later-onset pediatric multiple sclerosis (MS), whereas the effect of age on the CSF inflammatory profile is unknown and may contribute to delayed diagnosis.', 'To compare the CSF cellular and immunoglobulin G (IgG) profiles between earlier- and later-onset pediatric MS.', 'We queried the databases of 6 pediatric MS centers for earlier-onset (onset <11 years) and later-onset (> or = 11 and <18 years) patients with MS or clinically isolated syndrome who underwent CSF analysis within the first 3 months of presentation (observational study). We compared CSF white blood cell (WBC) differential count, IgG index, and IgG oligoclonal bands between age groups.', 'We identified 40 earlier-onset (mean age at onset = 7.2 +/- 2.7 years, 60% females) and 67 later-onset pediatric MS patients (15.1 +/- 1.7 years, 63% females). Although WBC count tended to be higher in earlier-onset patients (median = 9/mm(3) [0-343] vs 6 [0-140], p = 0.15), they had a lower proportion of lymphocytes (70% [0-100] vs 93% [0-100] of WBCs, p = 0.0085; difference = +3% per 1-year increase of age, p = 0.0011) and higher proportion of neutrophils than later-onset patients (0.5% [0-75] vs 0% [0-50] of WBCs, p = 0.16; difference = -1% per 1-year increase of age, p = 0.033). In earlier-onset disease, fewer patients had an elevated IgG index than in the later-onset group (35% vs 68% of patients, p = 0.031).'] Do younger children with MS have a distinct CSF inflammatory profile at disease onset?