AbstractImmunoreactive polymorphonuclear leukocyte elastase (iPMN elastase) was measured by ELISA in 36 patients with Kawasaki disease and in 41 patients with anaphylactoid purpura and other infectious diseases of childhood. The iPMN elastase level was very high in Kawasaki disease compared with other diseases. There was no significant correlation in iPMN elastase levels in Kawasaki disease between the complicated and non‐complicated groups in the acute period. The iPMN elastase in the non‐complicated group fell below 250 4g/1 at 20 days from the onset of the illness. However, iPMN elastase in the complicated group (coronary aneurysm or dilatation) tended to remain high above 500ligjlat 20 days from the onset. We suggest that increased iPMN elastase levels in Kawasaki disease reflect stronger enhancement and stimulation of PMN than in other disea