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A case of idiopathic giant atrium is described. The patient died of cardiac cachexia at the age of 80 years. Although she was diagnosed as having atrial fibrillation and cardiomegaly at the age of 40 years, she had not complained of any symptoms and did not receive any medical treatment. Echocardiograms and magnetic resonance imaging (upper panel) revealed pronounced bilateral atrial dilatation and normal wall motion of both ventricles. There were no structural abnormalities of the atrioventricular valves. In the compensated state of congestive heart failure, the plasma concentration of atrial natriuretic peptide (ANP) was slightly raised at 64 pg/ml (normal < 40 pg/ml). Follow up echocardiograms showed that the dimensions of both atria increased gradually, but there were no significant changes in ejection fraction, or left ventricular dimension and wall thickness. Necropsy showed pronounced dilatation of both atria without abnormal findings in the ventricles. Microscopic findings showed degeneration of myocardial fibres without infiltration of lymphocytes, with only a few myocytes remaining in both atria. Staining with Azan revealed remarkable fibrosis in both atria, but no significant abnormalities in the ventricular walls. Staining with an anti-ANP antibody revealed no ANP containing cells in both ventricles, but notable staining in both atria (lower panel). In idiopathic giant atrium, although the atrial myocytes retain their capacity for ANP secretion, the myocytes are replaced with fibrous tissue and the amount producing ANP are attenuated.