Our experience is based on 323 surgical operations on the mitral valve. Prosthetic valves were substituted in 308 patients and in 15 an anulo-valvulo-plastic operation. Mitral valve operation was associated with the correction of other heart lesions in 132 patients and in 197 patients, (objects of this study) surgery was carried out on the left atrio-ventricular valve. A detail report of the types of lesions found is made: 78 patients had massive calcification and 15 had a process of acute and subacute endocarditis. Mechanical prosthesis (Starr-Edwards 51; Björk-Shiley 77) were used 128 times and biologic prosthesis (Hancock 44; Carpentier-Edwards 7; Angell-Shiley) 54 times. Preference was given to the use of porcine prosthesis in patients presenting generic or specific controindications to anticoagulant therapy. Total mortality rate was 9,9% and in the last two years this fell to 5%. In our opinion, the sharp fall in mortality could be attributed to refined technic, better selection of cases, better myocardial protection consequent to the clinical use of cardioplegia and hypothermia.