Myxomatous lesion of the mitral valve, a major pathological factor in mitral valve prolapse (MVP), was analyzed in 33 cases with idiopathic MVP, 34 with secondary MVP, 30 with hypertension, 11 with right ventricular hypertrophy, and 28 control subjects. Area of the myxomatous lesion expressed as a percent area in the cross-section of each valve leaflet was obtained in each zone. With the increasing age, the % area increased particularly in the posterior leaflet and the area expanded from the rough zone to the basal zone. Compared with other conditions, the mitral valve in idiopathic MVP had extensive myxomatous lesion of 50% or more in the % area. Histologically, this area showed diffusely distributed lesion from the fibrosa to the ventricularis, disappeared collagen fibers, and disrupted elastic fibers irregularly located. Electron-microscopically, these disruption or disappearance were replaced by young mesenchymal cells. It was concluded that the increase in a myxomatous lesion is the essential pathological finding in idiopathic MVP, but this lesion may be developed secondarily in other conditions, though not so extensive.