This three-part article examines the histologic and morphologic basis for stenotic and purely regurgitant tricuspid valves. In Part I, conditions producing tricuspid valve stenosis are reviewed. In over 90% of stenotic tricuspid valves, the etiology is rheumatic disease. In isolated tricuspid stenosis, the etiology is either carcinoid or congenital. Rare causes of tricuspid stenosis include active infective endocarditis, metabolic or enzymatic abnormalities (Fabry's, Whipple's disease), and giant blood cysts.