We present a subgroup analysis of the first double-blind, placebo-controlled, randomised trial with vitamin D3 in MS. In the overall study population, there were 34 patients in the vitamin D arm and 32 patients in the placebo arm. All the patients were using interferon-β-1b (IFNB) therapy. The subgroup consisted of 15 patients in the vitamin D arm and 15 patients in the placebo arm, who had either at least one relapse during the year preceding the study or enhancing T1 lesions at the baseline MRI scan. We measured the total number of MRI T1 enhancing lesions, the number of new/enlarging T2 lesions and T2 lesion volume (BOD) (mm(3)), EDSS (Expanded Disability Status Scale), annual relapse Rate (ARR), timed 25-foot walk (T25FW), and timed 10-foot tandem walk (TT10W) at baseline and at 12 months in the vitamin D-treated and in the placebo-treated patients. There was a statistically significant reduction in the number of T1 enhancing lesions, a smaller T2 lesion volume growth and less new/enlarging T2 brain MRI lesions in the vitamin D3-treated than in the placebo-treated subgroup patients. The MRI results were slightly more pronounced in the subgroup than in the overall study population.