Thirteen patients with mild mitral stenosis and 21 asymptomatic patients after commissurotomy were studied by echocardiography in order to assess the rate of progression of mitral stenosis and the incidence of restenosis after successful open mitral commissurotomy. In the group with mitral stenosis there was a decrease of the diastolic closing velocity (E-F slope) from 35.7 to 29.5 mm./sec. (p less than 0.0005) over a period of 37 months. In 23% of the patients the stenosis increased significantly (p less than 0.0005) by echocardiographic parameters. Forty-eight months after commissurotomy we noted a significant over-all slowing of the diastolic closing velocity (from 52.6 to 44.8 mm./sec., p less than 0.0005) and a decrease of the mitral valve closure index DE/MAIC (from 1.7 to 1.5, p less than 0.0025). Five of 21 patients (24%) showed a change in one or both of these parameters which was greater than 2 standard deviations of the mean change. Based on echocardiographic criteria, we conclude that patients with mild mitral stenosis and asymptomatic patients following successful commissurotomy need only be checked approximately every 3 years.