Left isomerism with critical arrhythmia in a 34-year-old housewife is reported. Left isomerism is rare among adult congenital disorders. The prognosis in childhood depends on intractable heart failure, but is unclear in adulthood. We describe an adult patient with left isomerism who recovered from life-threatening arrhythmias. Transesophageal echocardiography was especially useful to identify the morphology of both atrial appendages.