Crohn's disease prevalence increases with increasing latitude. Because most vitamin D comes from sunlight exposure and murine models of intestinal inflammation have demonstrated beneficial effects of 1,25-(OH)2 vitamin D treatment, we hypothesised that Crohn's disease activity is associated with low vitamin D levels. In a cross-sectional study of 182 CD patients and 62 healthy controls, we measured serum 25-OH vitamin D. Stratified analysis was used to compare 25-OH vitamin D levels with Crohn's disease activity index, C-reactive protein, smoking status, intake of oral vitamin D supplements and seasonal variation in CD patients and healthy controls. Serum 25-OH vitamin D was inversely associated with disease activity: Median 25-OH vitamin D levels of Crohn's disease in remission, mildly, and moderately active diseases evaluated by Crohn's disease activity index were 64, 49, and 21 nmol/l (p<0.01) and by CRP 68, 76, and 35 nmol/l (p<0.05), respectively. Patients who took oral vitamin D supplementation had lower Crohn's disease activity index (p<0.05) and C-reactive protein (p=0.07) than non-users. Crohn's disease patients who smoked had lower vitamin D levels (51 nmol/l) than patients who did not smoke (76 nmol/l), p<0.01. Overall, Crohn's disease patients did not differ from healthy controls regarding 25-OH vitamin D levels. Active Crohn's disease was associated with low serum 25-OH vitamin D. Patients who smoked had lower 25-OH vitamin D levels than patients who did not smoke, independently of disease activity. Copyright © 2013 European Crohn's and Colitis Organisation. Published by Elsevier B.V. All rights reserved.