BACKGROUND Vitamin D has immune-regulatory functions in experimental colitis, and low vitamin D levels are present in Crohn's disease. OBJECTIVE To assess the effectiveness of vitamin D3 treatment in Crohn's disease with regard to improved disease course. METHODS We performed a randomized double-blind placebo-controlled trial to assess the benefits of oral vitamin D3 treatment in Crohn's disease. We included 108 patients with Crohn's disease in remission, of which fourteen were excluded later. Patients were randomized to receive either 1200 IU vitamin D3 (n = 46) or placebo (n = 48) once daily during 12 months. The primary endpoint was clinical relapse. RESULTS Oral vitamin D3 treatment with 1200 IU daily increased serum 25OHD from mean 69 nmol/L [standard deviation (s.d.) 31 nmol/L] to mean 96 nmol/L (s.d. 27 nmol/L) after 3 months (P < 0.001). The relapse rate was lower among patients treated with vitamin D3 (6/46 or 13%) than among patients treated with placebo (14/48 or 29%), (P = 0.06). CONCLUSIONS Oral supplementation with 1200 IE vitamin D3 significantly increased serum vitamin D levels and insignificantly reduced the risk of relapse from 29% to 13%, (P = 0.06). Given that vitamin D3 treatment might be effective in Crohn's disease, we suggest larger studies to elucidate this matter further. ClinicalTrial.gov(NCT00122184).