Mitral regurgitation is common in adults with aortic stenosis. When severe, it may aggravate the clinical condition and pose an additional therapeutic problem. The authors studied 40 consecutive patients with severe surgical aortic stenosis prospectively by transthoracic echocardiography and pre-operative transoesophageal echocardiography to determine the incidence, mechanism and degree of mitral regurgitation and its eventual relationship to the aortic stenosis. Mitral regurgitation was detected in all cases when both investigations were taken into consideration. It was usually mild, evaluated grade 2 by measuring the surface of the colour Doppler regurgitant jet, or mild to minimal of transoesophageal echocardiography in 35/40 patients (87.5% of cases). Rarely, a case of significant, autonomous mitral regurgitation (2 cases of valvular dystrophy, 1 pure severe mitral stenosis). On the other hand, calcification of the mitral annulus is common (14/40 patients, 35% of cases). The severity of the regurgitation in univariate analysis was significantly correlated mainly to the age of the patients (p = 0.027). The severity of the aortic stenosis (p = 0.0082) and the parameters related to the effects of stenosis, such as ventricular wall thickness and left atrial size. In multivariate analysis, the severity of the aortic stenosis and of its consequences were confirmed to play a role in the genesis of mitral regurgitation, the severity of which was correlated on transthoracic echocardiography to the aortic valve surface area and the left ventricular ejection fraction and, on transoesophageal echocardiography, to the transvalvular pressure gradient.