Fifty (male = 24; female = 26; age 49.33 +/- 12.16) presumably healthy adult Nigerians were prospectively examined for the presence of mitral valve prolapse (MVP). We performed clinical, electrocardiographic (ECG), M-mode echocardiographic (M-mode echo) and two-dimensional echocardiographic (2-D echo) examinations on these subjects. 2-D echos were obtained from parasternal and apical acoustic windows. Parasternal long axis view obtained when the transducer was perpendicular to the chest wall with both mitral valve leaflets and left atrium recorded was considered optimal for studying mitral valve systolic motion. MVP was defined as late or holosystolic bowing of mitral valve leaflet at least 2 mm or 3 mm, respectively, below the C-D line at M-mode echo; or, marked systolic extension of one or both mitral valve leaflets cephalad to the plane of mitral annulus into the left atrium. No subject had classical features of Marfan's Syndrome. Of the four subjects with cardiac symptoms, only one had diagnostic MVP. Three subjects had mid-to late systolic click following valsalva manouver. Seven subjects had apical late systolic murmur none of which was louder than grade II/VI. Four of them had combined anterior and posterior leaflet prolapse and one had posterior leaflet prolapse compatible with diagnostic MVP, thus resulting in 10% prevalence rate of MVP in the study population. Two other subjects with late systolic murmur had no echocardiographic evidence of MVP. Three subjects with non-diagnostic mild-to moderate prolapse of the anterior leaflet alone on 2-D echo had no clinical murmur even though two of them complained of palpitations. Seven otherwise normal subjects had holosystolic bowing of mitral valve leaflets on M-mode echo but not on 2-D echo and were thus classified into non-diagnostic MVP group. No subject with MVP had serious arrhythmias on resting ECG. These results indicate that the prevalence of MVP in presumably healthy adult Nigerians was 10%. The use of M-mode echo resulted in over-diagnosis, whereas 2-D echo was more accurate in identifying true anatomical and structural abnormalities of the mitral valve.