Forty consecutive patients undergoing mitral valve replacement with a giant left atrium, form the basis of this report. Two groups could be identified: (a) isolated giant left atrium (11 cases); (b) combined left and right atrial enlargement (29 cases). Tricuspid regurgitation was only detected in the latter group. The early hospital mortality was 20% and was related to the pulmonary arterial systolic pressure. A giant left atrium increased the operative mortality from 7% to 20% among patients submitted to mitral valve replacement in our unit (p less than 0.05).