To study the long-term overall effect of physical exercise on blood coagulation, 20 sedentary males and 15 sedentary females were trained three to four times a week with increasing intensity for 9 months. After 24 and 36 weeks all subjects ran a 15 km and a half-marathon (21 km) race, respectively. Blood samples were drawn before the training programme, 5 days before both races and 5 days after the half-marathon run. Plasma factor VIII coagulant activity and von Willebrand factor antigen concentration did not increase during the training programme. In both males and females plasma fibrinogen concentration was not enhanced after 24 weeks of training but increased in preparation for the 21 km race and was still raised significantly (P < 0.01) 5 days later. No significant changes in plasma thrombin-antithrombin III concentrations were observed in either group during the training programme. The results of this study demonstrate that an exercise programme of increasing intensity induces physical stress which has significant effects on plasma fibrinogen concentrations, even at rest. However, in contrast to acute post-exercise effects, a regular physical fitness programme does not induce a long-term activation of the haemostatic system.