Background/Aims: To evaluate the incidence rate of relapse in patients with inflammatory bowel disease (IBD) undergoing chondroitin sulphate (CS) treatment and its effect on the concentrations of several pro-inflammatory proteins. Methods: Prospective, observational, 12-month follow-up study in patients with IBD in remission, starting CS (Condrosan®, Bioiberica S.A.) treatment for osteoarthritis (OA). Crohn's Disease Activity Index and modified Truelove-Witts severity index were calculated for Crohn's disease and ulcerative colitis (UC) respectively. Levels of vascular endothelial growth factor (VEGFA), -C, fibroblast growth factor 2, hepatocyte growth factor, angiopoietin (Ang)-1, Ang-2, transforming growth factor beta, tumour necrosis factor alpha, interleukin (IL)-1β, IL-6, IL-12, IL-17, IL-23, intracellular adhesion molecule-1, vascular adhesion molecule-1, matrix metalloproteinase-3 and PGE2 were quantified by ELISA. OA joint pain was evaluated using a visual analogue scale. Results: A total of 37 patients (19 UC and 18 Crohn's disease) were included. The mean values for OA joint pain decreased after 12 months from 5.9 ± 2.8 to 3.0 ± 2.3 (p < 0.05). Only 1 patient (with UC) flared during follow-up. The incidence rate of relapse was 3.4% per patient-year of follow-up. Mean serum VEGFA levels increased between baseline (492 pg/ml) and 12-month treatment (799 pg/ml; p < 0.05). Conclusion: The incidence of IBD relapse in patients under CS treatment was lower than that generally reported. This treatment might modulate VEGFA. CS decreases OA-related pain in patients with IBD.