Patients with inflammatory bowel disease (IBD) frequently suffer from thromboembolic events. Anti-cardiolipin (aCL) antibodies have been shown to be associated with thrombosis. Recently, the antibodies against the anti-cardiolipin cofactor beta2-glycoprotein I (a(beta2)GPI) have been found with higher specificity for thrombosis. The presence of these antibodies was assessed in 128 patients with IBD [83 with ulcerative colitis (UC) and 45 with Crohn's disease (CD)] and 100 healthy controls (blood donors). Patients with UC and CD had a significantly higher prevalence of aCL (18.1% and 15.6%, respectively) than healthy controls (HC) (3%). Eleven IBD patients (8.6%) but no HC had a(beta2)GPI. None of the IBD patients with a history of thrombosis had aCL and only one of them (a UC patient with deep vein thrombosis of the right leg) had a high titer of IgG a(beta2)GPI. In conclusion, these data show that both aCL and a(beta2)GPI are significantly associated with IBD but further studies are needed to determine the significance of our findings.