Of multiple etiologies for pure tricuspid regurgitation, the causes may be divided into those associated with anatomically abnormal and anatomically normal tricuspid valves. Conditions associated with anatomically abnormal valves include rheumatic, floppy, Ebstein's anomaly, carcinoid, infective endocarditis, papillary muscle dysfunction, and other rarer causes such as radiation, hypereosinophilic syndrome, and endomyocardial fibrosis. Conditions associated with anatomically normal tricuspid valves include multiple causes of cor pulmonale (chronic obstructive pulmonary disease, primary pulmonary hypertension, and pulmonary hypertension due to mitral stenosis). Of 45 necropsy patients with clinically documented pure tricuspid regurgitation, conditions producing pulmonary hypertension (cor pulmonale, mitral stenosis) were the most frequent etiology (47 percent), followed by floppy (16 percent), rheumatic (11 percent), idiopathic dilated cardiomyopathy (9 percent), Ebstein's anomaly (7 percent), and miscellaneous conditions (10 percent). Of multiple causes of pure mitral regurgitation, tricuspid valve anular circumference and leaflet area are useful measurements in establishing etiology: dilated anuli (greater than 12 cm) are associated with Ebstein's anomaly, floppy valves, and multiple causes of pulmonary hypertension. Increased leaflet areas are associated with floppy valves and Ebstein's anomaly. Anular insertion site separates floppy tricuspid valves from Ebstein's anomaly. Of 21 purely regurgitant operatively-excised tricuspid valves, the etiologies were: rheumatic (43 percent), "functional" due to pulmonary hypertension from mitral stenosis (38 percent), Ebstein's anomaly (9 percent), infective endocarditis (5 percent), and floppy (5 percent). In contrast to etiology of pure mitral regurgitation, rheumatic disease is the leading cause for operative excision of purely regurgitant tricuspid valves. Comparison of systolic pulmonary arterial pressures and tricuspid valve morphology disclosed that pulmonary arterial pressures greater than or equal to 55 mm Hg and tricuspid valve anuli greater than 12 cm were associated with anatomically normal tricuspid valves and that pulmonary arterial pressures less than or equal to 40 mm Hg and tricuspid anuli less than 12 cm were associated with anatomically abnormal tricuspid valves.