The pathogenesis of the floppy valve syndrome is not fully solved. An almost invariable histological finding is the great accumulation of mucinous material in the valve leaflets and constitutes the basis of the valvular theory of the syndrome. The presence of a mucinous layer in normal valves-- the zona spongiosa--is not well recognised. To establish the normal range of the extent of this zone, 50 excised mitral valves from patients aged 2 to 89 years and who died as a result of road traffic accidents or non-cardiac causes have been analysed by measuring the thickness of the zone in relation to the valve thickness. A range of 0 to 60 per cent was found and this was not influenced by age. The findings were compared with 50 patients clinically diagnosed as suffering from the floppy valve syndrome. A value of over 60 per cent (range 62 to 94%) was found in 43 patients. The increase in the extent of the mucinous material was considered to be a secondary change in the thickened fibrosa which normally accompanies the floppy valve syndrome. Measurements of zona spongiosa falling within the normal range were found in seven patients. The clinical features, complications, and accompanying conditions have also been analysed. Chordal rupture had occurred in 20 patients, infective endocarditis in three, and calcification was found in four valves. In four patients the aortic valve was also involved and accompanying aortic root dilatation in an additional patient. It is suggested that these patients should not be included in the group of Marfan's forme fruste, nor in the typical floppy mitral valve syndrome. Apart from the valvular theory, the myocardial theory in the pathogenesis of the syndrome has been discussed and the components ensuring normal mitral valve function have been reviewed. It is concluded that an inherent, prominent zona spongiosa predisposes to the floppy valve syndrome, particularly if any one of the components of normal valve function is abnormal.