In this study we attempted to investigate the importance of posterior leaflet preservation during mitral valve replacement (MVR). One hundred and forty randomly selected patients with isolated mitral insufficiency were studied, half of whom had MVR with preservation of the posterior leaflet (Group I), whereas in the other half conventional MVR was performed, without preservation of the posterior leaflet (Group II). Within these two groups there were no in hospital deaths, but 5 patients in Group II were admitted to the hospital with congestive heart failure and 3 of them died after approximately 6 months. Long term mortality rate was 4.2% in Group II. Ejection fraction (EF) decreased postoperatively from 56% to 50% in Group I (p less than 0.05), and from 59% to 49% in Group II (p less than 0.05). Fifteen patients in each group and aged below 30 with sinus rhythm and in the first functional capacity according to the New York Heart Association classification (NYHA) were subjected to exercise study, 18 months after the operation. After exercise EF increased from 47% to 64% in Group I and decreased from 51% to 47% in Group II. These findings suggested that MVR with the preservation of the chordae tendineae can be done with a low morbidity and mortality rate in the early and late postoperative period.