To determine whether the fibrinolytic system is related to the occurrence of cardiac complication in Kawasaki disease, we measured tissue plasminogen activator, plasminogen activator inhibitor-1, and related factors in the plasma of children with Kawasaki disease. Patients (mean age, 1.8 years) were classified into patients with cardiac complication (n = 9) and no complication (n = 14) groups echocardiographically. They underwent single, high-dose, intravenous γ-globulin infusion therapy. Blood was drawn just before and the day after the single high-dose intravenous γ-globulin infusion therapy (acute phase), and at early and late convalescent phases. Leukocytosis was normalized immediately after the single, high-dose, intravenous γ-globulin infusion therapy. C-reactive protein and fibrinogen were increased in the acute phase and normalized by convalescent phases. D-dimer fraction of fibrin degradation products changed in a similar manner. Tissue plasminogen activator and plasminogen activator inhibitor-1 were increased in acute phase. The tissue plasminogen activator/plasminogen activator inhibitor-1 ratio was lower in the complication group than in the no complication group throughout the observation period (0.095 versus 0.208 after single, high-dose, intravenous γ-globulin infusion therapy, p = 0.006; 0.094 versus 0.183 at late convalescent phase, p = 0.024). A low tissue plasminogen activator/plasminogen activator inhibitor-1 ratio can predict the propensity for cardiac complication, and can help the physician to decide whether additional therapies are necessary in acute phase Kawasaki disease.