In a prospective study, clotting parameters of 37 children and adolescents with insulin-dependent diabetes mellitus (type 1 diabetes) were compared with those of a healthy control group. In a longitudinal follow-up over two years we found no statistical difference for most of the coagulation parameters investigated, including single factor analysis and coagulation inhibitors. The duration of type 1 diabetes was of no influence on these parameters. The only difference we found between patients and healthy controls was an elevation of PAI-1 in diabetics: median for PAI-1: 2.12 IU/ml in diabetics (range 0.50-8.40 IU/ml) and 0.84 IU/ml in normal controls (range 0.50-1.78 IU/ml). This difference was of statistic significance (p < 0.002) and also found in newly diagnosed patients. During observation time, none of our patients developed thrombosis or signs of vascular disease. In conclusion, we could not confirm the development of a hyper-coagulable state in pediatric diabetics, as it is described for adults with type 1 diabetes mellitus.