Diabetes mellitus involves changes in haemostasis which leads to the opinion that diabetes mellitus is a hypercoagulable state. However, little is known about the relationship of exercise and haemostasis in diabetics. Therefore, first of all the aim was to investigate if differences in blood coagulation and fibrinolysis can be demonstrated in subjects with insulin-dependent diabetes mellitus (IDDM) compared to controls and secondly, if differences concerning exercise induced changes can be seen in diabetics. 16 moderately fit subjects with IDDM and 16 matched controls underwent a maximal step test. Blood samples were taken after a 30 min rest, immediately and 1h after exercise and in addition after 30 min rest 7 days later at the same time of day. The rest values (mean of the two rest samples) in extrinsic total thrombin potential (TTPex, P=0.049), tPA-activity (P=0.007) were significantly higher and in PAI-1-antigen (P=0.002) -activity (P=0.049) lower in the diabetic group. APTT, PT, TAT (only control), TTPin, tPA-activity and -antigen and PAP were increased immediately and D-dimer (only control) 1 h after exercise, whereas PAI-1-activity and -antigen (only control) decreased immediately or 1 h after exercise (all minimal P<0.05). The increase of tPA-antigen and decrease in PAI-1-antigen after exercise were both lower in the diabetics (P<0.05). IDDM led to higher extrinsic total thrombin and fibrinolytic potential at rest, and reducing the exercise provoked distribution of tPA-antigen and decrease of PAI-1-antigen. Nevertheless a higher thrombotic risk after maximal exercise has not been investigated in young IDDM patients without complications and in good metabolic control.