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Background: Elevated antibody levels against Epstein–Barr virus (EBV) and a poor vitamin D status are environmental factors that may interact in relapsing-remitting multiple sclerosis (RRMS) aetiology. Objectives: To examine effects of high-dose oral vitamin D3 supplementation on antibody levels against EBV nuclear antigen 1 (EBNA1) in RRMS. Methods: Serum 25-hydroxyvitamin D3 (25(OH)D) and immunoglobulin G antibody levels against EBNA1 (whole protein and amino acid 385–420 fragment), EBV viral capsid antigen (VCA), cytomegalovirus (CMV) and varicella zoster virus (VZV) were measured in 68 RRMS patients enrolled in a 96-week randomised double-blinded placebo-controlled clinical trial of oral vitamin D3 supplementation (20,000 IU/week) (NCT00785473). Results: The mean 25(OH)D level more than doubled in the vitamin D group and was significantly higher than in the placebo group at study conclusion (123.2 versus 61.8 nmol/L, p < 0.001). Compared to the placebo group, both anti-EBNA1 protein and fragment antibody levels decreased in the vitamin D group from baseline to week 48 (p = 0.038 and p = 0.004, respectively), but not from baseline to week 96. Vitamin D3 supplementation did not affect antibodies against VCA, CMV or VZV. Conclusion: The results indicate that high-dose oral vitamin D3 supplementation can affect humoral immune responses against the latent EBV antigen EBNA1 in RRMS. [ABSTRACT FROM AUTHOR]