To elucidate the clinical features of mitral valve prolapse in apparently healthy young population, two-dimensional echocardiography was performed in the students (18-22 years) without documented organic heart diseases. Focusing on the systolic dislocation and configuration of the anterior mitral leaflet, a following two-dimensional echocardiographic criterion for grading prolapse was used: Grade I: subjects only with slight slip of the tip of the anterior mitral leaflet (AML) toward the left atrium, Grade II: those with considerable slip of the AML but keeping a normal convex shape in the leaflet body toward the left atrium, and Grade III: those with severe slip of the AML with its ballooning toward the left atrium. Among 2016 students examined, 1507 subjects (74.8%) were judged to be normal, 343 (17.0%) to be Grade I, 141 (7.0%) to be Grade II, and 25 (1.2%) to be Grade III. Of the 25 subjects in Grade III, 20 subjects underwent further examination including a questionnaire about the subjective complaints, physical examination, electrocardiograms at rest and during exercise, Doppler echocardiography and postural tests. Concerning the subjective symptoms, eight subjects had some complaints including chest pain, shortness of breath, dizziness, palpitation, fatigability and synocope, and four of the eight had more than three complaints. Mid-systolic click and a late systolic murmur were audible in four and funnel chest was observed in one. No specific findings were found by electrocardiograms. Mild mitral and tricuspid regurgitations were observed by Doppler echocardiography in four and nine subjects, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)