The potentialities of M-type echocardiography (EchoCG) for the evaluation of left ventricle function and the diagnosis of mitral regurgitation are studied in 76 patients with isolated, "pure" mitral insufficiency, confirmed at the clinic and by instrumental non-invasive and invasive examinations (right and left cardiac catherterization and left ventricle angiography). All EchoCG-alterations found have a non-specific character and are a manifestation of volume loading of the left ventricle and left auricle--increased volume and velocity of mitral blood flow. The presence of some of the following functional indices is of highest significance for EchoCG-diagnosis of mitral insufficiency: determination of regurgitation volume, regurgitation fraction and regurgitation index of left ventricle resp., growth of the telemetric size and volume, beat and minute volume of left ventricle, increased indices for the total and local contraction function of left ventricle, increased dimensions of left auricle, slightly elevated velocity and amplitude diastolic indices of anterior cusp of the mitral valve. The high-degree mitral insufficiency often masks EchoCG-manifestation of left ventricle contractility insufficiency. That is the reason why in case of significant mitral insufficiency, the normal or lightly diminished values of contractility indices--fraction of shortening, fraction of expulsion and average velocity of contraction of the circumferential fibres, are a manifestation of reduced contractability of left ventricle. The values of those EchoCG-indices significantly decrease only in mitral insufficiency with severe left ventricle dysfunction. EchoCG enables an approximate evaluation of the degree of mitral insufficiency, differentiating the light and moderate from severe mitral regurgitation with a relative accuracy. The extent of the regurgitation volume, fraction and index, the diastolic size and volume, beat and minute volume of left ventricle and the size of left auricle are of highest importance for the determination of the degree of mitral insufficiency. With light mitral insufficiency, EchoCG could be absolutely normal.