Forty-two patients with systolic posterior motion of the mitral valve on M-mode echocardiogram were reevaluated after a mean clinical follow-up of 6.7 years and an echocardiographic follow-up of 5.1 years. A new method for quantitation of mitral valve prolapse (MVP) is presented. A straight line (A) was drawn connecting the site of posterior leaflet motion with the D point of the subsequent diastole. A vertical line (B), termed "the degree of MVP," was drawn from the site of maximal MVP to line A. The degree of MVP was then correlated with various clinical and ECG parameters and with the presence of complications. Measurements could be taken in 21 patients; the mean degree of MVP increased from 5.9 +/- 2.1 to 7.1 +/- 2.9 mm (P less than 0.01), and mean left atrial size increased from 3.0 +/- 0.5 to 3.5 +/- 0.8 cm (P less than 0.01). No correlation was found between the degree of MVP and symptoms, auscultatory findings or the presence of complications. There were no deaths and in none was life-threatening arrhythmia documented. The complications we encountered were endocarditis in five patients, transient ischemic attacks in two and cardiac arrest during surgery in one. We conclude that 1) the degree of MVP can be measured by M-mode echocardiography; 2) there seems to be no correlation between the degree of MVP and the patient's clinical state; 3) complications in MVP are rather frequent; and 4) left atrial size and degree of MVP tend to increase with time, which may indicate a progressive nature of the disease.