OBJECTIVES: The study objective was to correlate the degree of tricuspid regurgitation with clinical indicators of right-sided heart failure and both qualitative and quantitative measures of right-sided heart morphology and function in patients with degenerative mitral valve disease. METHODS: From 2001 to 2007, 1833 patients with degenerative mitral valve disease, structurally normal tricuspid valve, and no coronary artery disease underwent surgery. Right-sided heart morphology (right ventricular base-to-apex length, tethering distance and area, and right atrial systolic area) and right ventricular function (tricuspid annular plane systolic excursion, myocardial performance index, and tricuspid valve annular shortening) were measured on preoperative transthoracic echocardiograms for 100 randomly selected patients from each of tricuspid regurgitation grades 0, 1+, and 2+, and for all 93 patients with tricuspid regurgitation grade 3+/4+. Multivariable regression was used to evaluate the association of left- and right-sided heart morphology and function with tricuspid regurgitation. RESULTS: Increasing tricuspid regurgitation grade was associated with higher right ventricular pressure (P < .0001), increased tethering distance (P = .008), larger right atrial size (P = .0002), and worsening right ventricular function, particularly when 3+/4+ tricuspid regurgitation was present. When tricuspid regurgitation was 3+/4+, both tricuspid annular plane systolic excursion and myocardial performance index were almost certainly abnormal. Changes in right-sided heart morphology and right ventricular dysfunction were synergistic in relation to severity of tricuspid regurgitation. CONCLUSIONS: Functional tricuspid regurgitation accompanying mitral valve disease is associated with proportional changes in right-sided heart morphology; however, severe tricuspid regurgitation is nearly always associated with right ventricular dysfunction, suggesting a synergistic relationship. Right ventricular dysfunction is likely as important as tricuspid regurgitation because it offers an explanation for the negative prognostic impact of tricuspid regurgitation and has implications for the clinical management of patients.