Background: We investigated whether and to what extent the ratio between circulating fibrinogen (Fg) and its degradation products (FDP)reflects the severity of coronary artery disease (CAD) in type 2 diabetic patients.Methods: Plasma levels of Fg and FDP were determined, and Fg/FDP ratio was calculated in 344 consecutive patients with type 2 diabetes and chest pain on exertion undergoing coronary angiography.The severity of CAD was evaluated by the number of significant CAD (>50% luminal diameter narrowing) and Gensini score.Results: Plasma Fg was higher, but Fg/FDP ratio was lower in patients with significant CAD (n =255) compared with those without (n =89),due to a disproportionate increase in FDP.Fg and FDP correlated positively, while Fg/FDP ratio negatively, with the number of diseased coronary arteries and the tertile of Gensini score (all P values for trend < 0.01).After adjusting for age, sex, risk factors for CAD, lipid profiles, glycosylated hemoglobin A1c, creatinine, leukocyte count, and high-sensitivity C-reactive protein, Fg/FDP ratio remained an independent determinant for multivessel coronary disease (MVD) (odds ratio [OR], 0.869;95% confidence interval [CI], 0.788-0.958,P =0.005) and high tertile of Gensini score (OR, 0.797, 95% CI, 0.682-0.930, P =0.004).The area under the curve of Fg/FDP ratio was larger than that of Fg for predicting the presence of MVD (0.647 vs.0.563, P =0.048) and Gensini score ≥ 30 (0.656 vs.0.538, P =0.026).Conclusions: Elevated plasma Fg and FDP level and reduced Fg/FDP ratio are associated with presence of CAD, and Fg/FDP ratio is superior to Fg in reflecting severe coronary atherosclerosis for patients with type 2 diabetes.