Introduction: Acute massive mitral insufficiency due to rupture of chordae tendineae is a rare but sometimes fatal condition in children. Serious symptoms such as dyspnea or cyanosis suddenly appear in healthy children. Although early diagnosis and appropriate medical and surgical treatments are necessary, this disease has not been well summarized in literatures and has not been well recognized by general pediatricians, either. Since the number of reported cases has been recently increasing in Japan, we have developed a nationwide survey of the disease. Hypothesis: We assessed the hypothesis that a Japanese nationwide survey clarifies etiology, clinical characteristics, treatments, and prognosis of the disease. Method: A questionnaire was sent to 511 pediatric institutes in Japan to unveil clinical and laboratory findings, surgical procedures, and prognosis of the patients during the recent 10 years. Results: A total of 88 children's data were analyzed. Males and females were 49 and 39, respectively. The number of patients was increasing during the recent 5 years (peaked in 2009, 17 cases). The age at onset mainly ranged from 4 to 6 months (68 cases, 77%) with a median at 5 months. The average body weight at birth and at onset was 2.97+/-0.48kg and 6.83+/-1.19kg, respectively. Seven patients had a history of Kawasaki disease. Maternal-derived serum anti-SSA antibody was detected in 2 patients. Average values of WBC, CRP, and cardio-thoracic ratio at diagnosis were 17,040+/-8,210/uL and 2.53+/-2.83mg/dl, and 55.4+/-5.8%, respectively. Forty-five patients (51%) underwent mitral valve repair and 25 patients (28%) underwent mitral valve replacement. Histopathological examinations of the mitral tissue were performed in 21 cases, showing mononuclear cell infiltration in 10 cases (48%) and myxoid change in 7 cases (33%). Six patients (6.8%) died before (4 cases) and after (2 cases) surgical operation. Moderate to severe brain damage remained in 10 patients (11%). Conclusions: Acute rupture of mitral chordae tendineae affects infants with high rates of mortality and morbidity. Since the incidence is recently increasing, precise recognition, early diagnosis, and appropriate medical and surgical treatments are strongly recommended.