Several fibrinolytic variables, including plasminogen activator inhibitor activity, were studied before and after exercise in 67 normolipidaemic patients with coronary artery disease and in 25 hyperlipidaemic patients with coronary artery disease. Before exercise plasminogen activator inhibitor activity was higher in the patient groups than in a group of 10 healthy volunteers. For those who were normolipidaemic plasminogen activator inhibitor activity was greater in patients with angina pectoris who had had a myocardial infarction. The concentration of antigenic tissue-type plasminogen activator was similar in all the patients with coronary artery disease and higher than in the control group. After the exercise test fibrinolytic capacity was lower in the patients with angina pectoris and a previous history of myocardial infarction. After exercise both the released immunological tissue-type plasminogen activator and fibrinolytic capacity were lower in the hyperlipidaemic patients than in the normolipidaemic patients. The concentration of plasminogen activator inhibitor was also higher in the hyperlipidaemic patients. Patients with hyperlipidaemia IV had the highest plasminogen activator inhibitor activity. The increase in plasminogen activator inhibitor activity found in the patients was partially inhibited by antiserum against plasminogen activator inhibitor-1 in vitro. The formation of a complex of about 115,000 daltons between plasminogen activator inhibitor and purified tissue-type plasminogen activator was detected by a zymographic fibrin technique. These findings show that in patients with coronary artery disease fibrinolytic activity is impaired by an increase in plasminogen activator inhibitor. Impaired fibrinolysis may be related to the clinical evolution of coronary artery disease in these patients.