High plasma fibrinogen levels are associated with vascular complications in the general population. Fibrin, the structural element in a clot, is derived from fibrinogen by activation of thrombin. An abnormal fibrin gel structure has been demonstrated in patients with myocardial infarction and in diabetic patients during poor metabolic control. In the present study the properties of fibrin gel structure were investigated in 20 patients with insulin-dependent diabetes mellitus (IDDM): 10 patients without (age: 30 ± 8; diabetes duration: 7 ± 6 years), and 10 patients (age: 44 ± 7; diabetes duration: 27 ± 9 years) with microangiopathy. Fifteen healthy subjects served as controls (age: 40 ± 8 years). The glycosylated haemoglobin level (HbA_1c) was elevated (p < 0.001) in the patients: 6.5 ± 1.5 % in diabetic patients without, and 7.1 ± 1.0 % in diabetic patients with microangiopathy. C-reactive protein and plasma fibrinogen were similar as compared to healthy control subjects. The properties of the fibrin gel structure; i. e. the permeability coefficient (Ks) and the fibre mass length ratio (μ) formed in recalcified plasma on addition of thrombin were investigated. Ks was decreased in the diabetic patients, with (6.5 ± 2.0 cm²; p < 0.01) and without microangiopathy (6.5 ± 2.7 cm²; p < 0.05), as compared to healthy subjects (10.0 ± 3.4 cm²), while μ was not significantly (p = 0.14) altered. The results indicate a lower fibrin gel porosity in patients with IDDM, despite normal plasma fibrinogen and irrespective of microangiopathy. The abnormal fibrin gel structure may be due to an increased glycosylation of the fibrin (-ogen) molecule caused by long-term hyperglycaemia and may be of importance for the development of angiopathy in diabetic patients. [Diabetologia (1996) 39: 1519–1523]