OBJECTIVE: To evaluate the determinants of elevated fibrinogen levels and the impact of hyperfibrinogenemia on vascular complications in diabetes. RESEARCH DESIGN AND METHODS: Plasma fibrinogen, glucose, HbA1, and lipids were measured in 116 ambulatory type I and type II diabetic patients with (n = 59) or without (n = 57) clinical evidence of micro- or macrovascular complications. In 56 of these patients, factor VII activity and CRP also were measured. Univariate and multivariate data analyses were conducted. RESULTS: Overall mean ± SE fibrinogen levels in patients (339 ± 7.3 mg/dl) were elevated markedly compared with control subjects (248 ± 9.1 mg/dl). Fibrinogen levels were elevated disproportionately in patients with type II diabetes (P < 0.0001), hypertension (P = 0.0001), obesity (P < 0.0001), and vascular complications (P < 0.0001). Fibrinogen was correlated significantly with age (P < 0.001), cholesterol (P = 0.002), CRP (P < 0.001), and factor VII activity (P = 0.032), but not with plasma glucose, triglycerides, HDL cholesterol, or disease duration. Stepwise multiple regression analyses revealed that type II diabetes and presence of vascular complications were major determinants of fibrinogen. For vascular complications, fibrinogen emerged as one of only three independent predictors, the other two being diabetes duration and hypertension.