Certain risk factors for myocardial infarction have been linked with disturbances in fibrinolytic activity. The recent development in our laboratory of new sensitive and specific methods for determination of tissue plasminogen activator (t-PA) activity and antigen, as well as the discovery of a new rapid inhibitor of this enzyme, enabled us to study fibrinolytic function in detail in a representative population of postinfarction patients. Seventy-one patients (62 men and 9 women) who had survived a myocardial infarction before the age of 45 were compared with 50 healthy subjects of similar age, three years after the infarction. Low t-PA activity after venous occlusion, mostly explained by high plasma levels of the t-PA inhibitor and to some extent by impaired release of t-PA from the vessel wall, was a frequent finding in the patients. The level of t-PA inhibitor was positively and significantly correlated with levels of serum triglycerides. Our data suggest that reduced fibrinolytic capacity due to increased plasma levels of a rapid inhibitor of t-PA may have pathogenetic importance in myocardial infarction, particularly in patients with hypertriglyceridemia.