We report a rare case of giant right atrial dilatation in a patient with Marfan's syndrome complicated by severe tricuspid regurgitation. Interestingly, the right atrial pressure was normal, but the atrial natriuretic peptide level was markedly elevated. We suggest that the underlying connective tissue abnormality had allowed extensive atrial distension which by causing an increased intramural tension according to Laplace's law stimulated atrial natriuretic peptide secretion.