An activated thrombogenesis has been reported in inflammatory bowel disease (IBD). In this study we evaluated whether a fibrin degradation product, the D dimer, is increased in patients with IBD. D dimer plasma levels were evaluated by sandwich ELISA in patients with Crohn's disease (24), ulcerative colitis (25), gastrointestinal (GI) disease controls (10), hospital controls (13) and healthy subjects (14). Circulating D dimer was significantly higher in ulcerative colitis (median 651; 95% CI 89-1275 ng/ml) than in healthy subjects (median 412; 95% CI 112-672 ng/ml; p = 0.002), Crohn's disease (median 466; 95% CI 6-931 ng/ml; p = 0.005) and GI disease controls (median 446; 95% CI 196-688 ng/ml; p = 0.023). In ulcerative colitis, plasma D dimer was related to inflammatory parameters such as C-reactive protein (p < 0.01) and seromucoids (p < 0.001). Circulating D dimer was age-related in all groups (p < 0.05). Fibrin degradation, as reflected by plasma D dimer, is detected in patients with ulcerative colitis exhibiting a marked acute phase response.