Plasminogen activator inhibitor (PAI) activity, an important regulator within the fibrinolytic system, has been shown to be a risk indicator for venous and arterial thrombosis. The present study aimed to test the effects of exercise training on PAI activity, and to link possible changes in PAI activity to changes in cardiovascular fitness, body composition, and the lipid profile. Four groups of previously sedentary subjects were studied thrice in an 8-month period. A long-term training group (N = 11) trained during the entire 8-month period. A detraining group (N = 14) trained for 4 months and then reverted to sedentary habits. A postponed training group (N = 16) trained only during the second 4-month period, and a no-training control group (N = 9) remained untrained throughout the entire 8-month period. PAI activity always decreased in response to training, but the training effects were small and spontaneous seasonal shifts in PAI activity of the control groups clouded their interpretation. Furthermore, detraining failed to influence PAI activity and training-induced changes in PAI activity were not related to simultaneous changes in maximal oxygen consumption, diastolic blood pressure, triglycerides, or percentage body fat, and inconsistently related to the training-induced changes in LDL-cholesterol and HDL-cholesterol. The occurrence of simultaneous changes in body fat, blood pressure, and the lipid profile underscores the potential of regular exercise to protect against cardiovascular disease. Whether these beneficial effects are accompanied by changes in the fibrinolytic system remains to be proven.