BACKGROUND: Multiple Sclerosis is associated with low serum levels of 25-hydroxyvitamin D (25(OH)D). We investigated the association between serum levels of 25(OH)D and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active metabolite, and clinical MS severity as expressed by EDSS-score and relapse rate. STUDY-DESIGN: Cross-sectional study. PATIENTS AND METHODS: Serum samples from 267 MS patients were collected for 25(OH)D and 1,25(OH)2D measurement. Clinical MS parameters at the date of serum sampling were determined. Results: Both metabolite levels were significantly lower in the progressive forms compared to the relapsing remitting (RR)MS phenotype. In RRMS patients (disease course < or = 5 years), high 25(OH)D levels were associated with a high chance of remaining relapse-free. Low 25(OH)D levels were associated with high EDSS-scores. 1,25(OH)2D was not directly associated with relapse rate or EDSS-score, and was dependent of age and 25(OH)D level. CONCLUSION: Serum levels of 25(OH)D were associated with both relapse rate and disability in MS patients. These results are suggestive for a disease modulating effect of the serum concentrations of 25(OH)D on MS. The low circulating 1,25(OH)2D levels in progressive MS are due to older age and lower 25(OH)D levels. The potential consequences for vitamin D supplementation in MS will be discussed.