This study concerns 120 patients with tricuspid incompetence (TI) combined with mitral or mitral-aortic valve disease. This first part is devoted to the 48 patients TI of whom was not corrected, and the long-term course (average 40 months) after mitral-aortic lesions correction was analysed. Assessment of the degree of TI at operation differed markedly from its pre-operative clinical assessment; surgical findings agreed with the clinical estimations in 10 of 12 cases of TI considered to be negligible, but only in 4 of 24 cases with a TI considered as intense clinically. The long-term unfavourable courses (TI with persistent right ventricular failure) were more common in the group of TI considered intense clinically (59%) than in the TI considered clinically as negligible (23%). Assessment by the surgeon during operation of the degree of TI seemed a less accurate element of valuation of the long-term prognosis. The mean pulmonary artery pressure measured at operation did not have, in the patients studied, a significant influence on the long-terme course after mitral-aortic valve correction. On the contrary, the results were less favourable in the group of patients whose TI was old-standing and in the group of patients with a cardio-thoracic ration higher than 0.65. It seems therefore legitimate to recommend correction of both the cases of TI considered to be intense by the surgeon, and those considered to be intense clinically, even if the surgeon finds them to be negligible. This attitude is the more justified the older the TI and the larger the heart enlargement.