Abstract Background Kawasaki disease (KD) is a self-limiting form of systemic vasculitis. KD usually occurs in infants and young children and is rarely seen in adolescents. On rare occasions, KD is accompanied with reduced organ perfusion due to systolic hypotension, a condition known as Kawasaki disease shock syndrome (KDSS). The multifactorial causes of KDSS may include intensive vasculitis with capillary leak, myocardial dysfunction, and release of proinflammatory cytokines. However, the mechanisms underlying the pathophysiology of KDSS have not been fully elucidated. Case summary A febrile 17-year-old male with cervical lymphadenopathy developed extreme shock with rapid cardiac dysfunction and reduced organ perfusion. Electrocardiogram revealed ST elevation in the precordial leads and increased serum levels of cardiac enzyme levels. Endomyocardial biopsy at the acute phase revealed CD3+, CD4+ or CD8+, and CD20− lymphocytes and CD68+ macrophages within infiltrates in the myocardium with mild interstitial fibrosis. He was treated with intravenous immunoglobulin (IVIG) and followed by glucocorticoids with mechanical circulatory support. His cardiac function recovered rapidly with no apparent adverse effects. Discussion Our results suggest that KDSS may be a form of myocarditis, a condition in which inflammatory cells infiltrate the myocardium. Early immunosuppressive therapy, including IVIG and glucocorticoid therapy, may limit the severity of disease and improve the prognosis. As shown by this case, an accurate diagnosis of KD and KDSS will lead to early intervention and improved prognosis even among those in an older cohort.