The paper reports an electrocardiographic and M and B-mode echocardiographical study in 3682 non-selected patients among which a total of 241 were identified with mitral valve prolapse. In males the highest frequency was found during the second decade of life with a tendency to decrease with increasing age. In females, who were found to be the most severely affected (2:1 in relation to males), two peaks of incidence were observed during the third and fifth decade, with a greater frequency in the latter and a marked absence of cases in the fourth decade. Many echocardiographically diagnosed mitral prolapses were both clinically and electrocardiographically silent; moreover it was observed that the risks of this complication increased in parallel to the thinness and redundance of the mitralic flap affected by prolapse and the dilation of the valvular ring.