Of the 255 patients under observation of the AA. with ostium secundum interatrial defect who underwent surgery, 23 (9%) presented an associated mitral regurgitation. All patients had a hemodynamic test and a complete contrahistography, 8 patients polygraphic and 5 patients echocardiographic examinations. The clinical data alone, ecgraphic and Rx, supplied sufficient indications for a correct diagnosis in 87% of the cases. 22 patients underwent surgery: the septal defect was corrected in all patients, mitral regurgitation only in 9; in 5 by valve substitution, in 4 through conservative techniques. Only one patient died of cerebral coma. A report is made on the data relative to the clinical history, Rx, ecgraphic and hemodynamic examinations of the 23 patients under observation, the various anatomosurgical aspects of the valvular alterations, and the criteria adopted for the surgical correction of the mitral regurgitation.