Elevated levels of anti-cardiolipin antibodies are associated with an increased risk for venous and arterial thrombosis. In patients with inflammatory bowel disease thrombosis is a well known complication. We determined the prevalence of elevated anti-cardiolipin antibodies in 136 patients with inflammatory bowel disease compared with 136 healthy controls and analyzed thromboembolic complications in patients with increased anti-cardiolipin antibody levels. Anti-cardiolipin antibody titers were significantly elevated in patients with Crohn's disease (5.7 units/ml) and ulcerative colitis (5.3 units/ml) compared to the control group (2.5 units/ml). We found no correlation between disease activity and anti-cardiolipin antibody levels. Seven patients had deep venous thrombosis in their history, in three of them this was complicated by pulmonary embolism. In only two of the seven patients with deep venous thrombosis were anti-cardiolipin antibody levels increased. In conclusion, anti-cardiolipin antibody titers were significantly increased in patients with inflammatory bowel disease. Elevated anti-cardiolipin antibody levels appear to play no role in the pathogenesis of thromboembolic events in patients with inflammatory bowel disease.