A woman (63 years old) is hospitalized in urgency (pulmonary oedema). The routine echocardiography shows: very important apical HCM (39mm), not-obstructive (basal portion of LV is dilated, walls thickness is normal); high fillings pressures; significant mitral regurgitation. Visualization in apical position apical is poor (important calcification of the hypertrophied muscle). The Simpson method is impossible because not-regular geometrical shape of LV cavity. The mechanism and the importance of the mitral regurgitation are difficult to evaluate. A dynamic acquisition by real time 3D echo (IE-33, Philips) is performed during this examination (from para-sternal window). The post treatment of these data (20mn) according to our standardized technique made it possible to determine with precision: the LVEDD in basal level (66mm); tele-diastolic (110ml) and tele -systolic (37ml) volumes, stroke volume (73ml) of LV (by discs summation method); EF (66%); surface (by direct planimetry) of LVOT (2.8cm²) and stroke volume ejected to the aorta (39ml); mechanisms of the mitral regurgitation (retraction of anterior portion of P2 scallop by direct attachments to the lateral wall of the LV and dilation of the mitral ring (46mm on 46mm); importance of the mitral regurgitation (important: regurgitation fraction=46%). Conclusion: In this rare, anatomically atypical and technically difficult case, the 3D echo realized by applying of the standardized navigation technique brings additional important information compared to the 2D echo, with an unequalled precision. The volumetric acquisition is carried out in real time; the time of postprocessing is acceptable for a generalized clinical application.