First described in 1856 as small, filiform, pale outgrowths from the free border of the cardiac valves, larger specimens can now be detected by echocardiography. We analysed 30 consecutive cases to define the anatomic of LE were determined macroscopically and histological examination was then carried out with six different stains. LE was found on 18 aortic, 10 mitral, 2 tricuspid and 2 pulmonary valves in the 30 patients examined. Their size ranged from 2 to 12 mm with 7 examples being over 5 mm long. Histologically there appeared to be two forms: mobile, pediculated lesions (20 cases) either with the usual changes of an acellular fibrous axis containing rings of granulous orcenophilic material or with more complex associations of these changes resulting in pseudo-papillomata of the valves; the second form of LE is incorporated in the valve itself. It thickens the valve and becomes covered by a layer of flattened endothelium. The histological study suggested several pathogenic mechanisms, all related to local conditions rather than to general circulatory changes: folding of the valvular endothelium onto itself followed by organisation into tubular structures filled with oedema and hyaklin, deposits of fibrin layers on zones of endothelial ulceration caused by trauma on the line of apposition of the cusps with partial detachment of fibrinous material which then endothelialises; finally, subendocardial incorporation of oedema, fibrin, and red blood cells. Although LE may occur in association with non-bacterial thrombolic endocarditis, these two conditions do not seem to be related.(ABSTRACT TRUNCATED AT 250 WORDS)