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Observational studies have suggested that vitamin D may reduce inflammation in relapsing-remitting multiple sclerosis (RRMS), but this has not been clearly confirmed in randomized controlled trials. To further explore the possible anti-inflammatory effects of vitamin D in RRMS, we examined the effect of high-dose oral vitamin D on eleven markers of systemic inflammation in 68 RRMS patients enrolled in a double-blinded randomized placebo-controlled trial of vitamin D supplementation (20,000 IU/week) (NCT00785473). Serum inflammation markers and 25-hydroxyvitamin D (25(OH)D) were measured at baseline and week 96, and no restrictions were set on additional standard immunomodulatory treatment for RRMS. The mean 25(OH)D level rose from 56 ± 29 to 123 ± 34 nmol/L among patients receiving vitamin D supplementation, whereas only a minor increase from 57 ± 22 to 63 ± 24 nmol/L was seen in the placebo group. However, no significant differences appeared between the vitamin D group and the placebo group for any of the inflammation markers. Patients on immunomodulatory therapy had significantly higher levels of interleukin-1 receptor antagonist and chemokine (C-X-C motif) ligand 16 than patients without immunomodulatory treatment, but there were no clear synergistic effects between immunomodulatory therapy and vitamin D supplementation on any of the inflammation markers. The rise in 25(OH)D levels after vitamin D supplementation was unaffected by immunomodulatory treatment. We conclude that in this study of RRMS patients, high-dose oral vitamin D supplementation prominently increased serum 25(OH)D levels without affecting markers of systemic inflammation, while a more anti-inflammatory phenotype was found among patients on immunomodulatory treatment. [ABSTRACT FROM AUTHOR]