The clinical diagnosis of the presence and severity of tricuspid insufficiency is difficult and is complicated by the transient nature of this abnormality as well as its usual occurrence in subjects with other valve disease. This paper presents correlative hemodynamic data from 100 consecutive catheterized patients with valvular heart disease in 90 of whom the presence of tricuspid insufficiency was sought by the indicator-dilution technic. Tricuspid regurgitation was present in 28 of the 90 patients. Subjects were grouped into 20 controls without tricuspid insufficiency and those with mild, moderate, and severe tricuspid insufficiency. The hemodynamics of these groups were compared by the analysis of variance to determine which parameters relate most closely to tricuspid insufficiency. The results are interpreted to confirm that tricuspid insufficiency results from those factors which overdistend the right side of the heart, and by permitting blood to escape retrograde from the right ventricle serves as a safety valve preventing progressive overload of the pulmonary circulation.