M-mode echocardiographic studies of left ventricular function in mitral valve disease were evaluated in 19 surgically treated patients before and one year after operation (mean 12.7 months). Twelve patients had mitral stenosis (MS) and seven patients had mitral regurgitation (MR). Before surgery, average end-diastolic and end systolic dimensions (EDD, and ESD) and left atrial dimension were significantly greater than normal in subject with MR. After surgery, EDD fell significantly from $66.5{pm}8.4$ (SD)mm to $52.7{pm}6.3$mm (P 0.01) at the time of late follow up study; ESD fell significantly from $46.5{pm}9.7$mm to $36.4{pm}8.6$ (P 0.05) on early follow up study; left atrial dimension fell significantly from $60.5{pm}6.8$mm to $48.1{pm}7.2$mm (P 0.01) at the time of the late follow up study. In patients with MS, EDD and ESD were normal and did not change significantly at any time after surgery. The left ventricular ejection fraction (E.F.) was normal in both groups. preoperatively([MR: $64.2{pm}15.1$, MS: $65.7{pm}12.3$). After surgery, E.F. did not change significantly at any time after surgery in both groups, but de-creased from $64.2{pm}15.1$% to $59.5{pm}11.2$% in MR patients at the time of early follow up study.