Rare cases of macrophage activation syndrome (MAS) occurring during the acute phase of Kawasaki disease (KD) have been reported. We sought to characterize, review treatment, and outcomes of KD patients with clinical features of MAS. Medical histories of patients treated for KD and MAS between January 2001 and March 2008 were reviewed. Of 638 KD patients seen, 12 (1.9%) had additional clinical findings usually associated with MAS; 7 of them were males older than 5 years (6.1%; odds ratio: 6.8, P=0.002). Clinically, 9 patients had at least 4 of 5 KD clinical signs, and all patients had prolonged fever beyond initial intravenous immunoglobulin treatment. Hepatosplenomegaly, cytopenia in two or more cell lines, hypertriglyceridemia and/or hypofibrinogenemia, and increased D-dimers were seen in 11 patients. Hyperferritinemia and elevated hepatic enzymes were seen in all patients. Four patients had biopsy-proven evidence of hemophagocytosis. All but 2 patients met at least 5 of 8 criteria necessary for MAS diagnosis. Treatment beyond the standard KD protocol (aspirin + intravenous immunoglobulin) was necessary in all but 1 patient. All patients eventually recovered with no long-term sequelae. A high index of suspicion for clinical features associated with MAS is warranted for KD patients to provide appropriate and timely treatment.