Morphologic comparison of patients with mitral valve prolapse who died suddenly with patients who died from severe valvular dysfunction or other conditions. Clinical and necropsy findings are described in 56 patients with mitral valve prolapse: 15 patients, aged 16 to 69 years (mean 39), died suddenly and mitral valve prolapse was the only cardiac condition found at necropsy (hereafter called isolated mitral valve prolapse); the remaining 41 patients had other conditions that were capable of being fatal. Of the latter 41 patients, 7, aged 17 to 59 years (mean 45), had associated congenital heart disease, and 34 patients, aged 17 to 70 years (mean 52), had no associated congenital cardiac abnormalities. Compared with the 34 patients without associated congenital heart disease and with nonmitral valve prolapse conditions capable in themselves of being fatal, the 15 patients who died suddenly with isolated mitral valve prolapse were younger (mean age 39 +/- 17 versus 52 +/- 15 years; p = 0.01), more often women (67% versus 26%; p = 0.008) and had a lower frequency of mitral regurgitation (7% versus 38%; p = 0.02). The 15 patients dying suddenly with isolated mitral valve prolapse also were less likely to have evidence of ruptured chordae tendineae (29% versus 67%; p = 0.04). The frequency of increased heart weight (67% versus 59%), a dilated mitral valve anulus (80% versus 81%), a dilated tricuspid valve anulus (17% versus 17%), an elongated anterior mitral leaflet (86% versus 54%), an elongated posterior mitral leaflet (79% versus 77%) and fibrous endocardial plaque under the posterior mitral leaflet (73% versus 63%) was similar between the two groups. The severity of the prolapse (mild 20% versus 11%; moderate 27% versus 58%; severe 53% versus 32%) also was similar between the two groups. Thus, persons with mitral valve prolapse dying suddenly without another recognized condition tend to be relatively young women without mitral regurgitation. 