Isolated tricuspid regurgitation (TR) is rare. Generally, TR is caused by pulmonary hypertension secondary to mitral or aortic valve disease, commonly referred to as "functional" regurgitation. The causes of isolated TR in adults include trauma, endocarditis, carcinoid heart disease, and congenital malformation of the tricuspid valve apparatus. In addition, isolated TR should be distinguished from Ebstein anomaly. In the present case, the patient had no definite causes of TR, and neither mitral nor aortic valve disease. The tricuspid valve of this patient showed no abnormalities other than a severely dilated tricuspid annulus. Isolated TR caused by annular dilatation was diagnosed and then ring annuloplasty was perfomed. The subsequent clinical course was satisfactory.