SUMMARY We examined the long-term effects of closed instrumental mitral commissurotomy on mitral valve area (MVA) in 18 patients, followed for as long as 14 years after successful operation. Each patient had preoperative and early postoperative cardiac catheterization; a late postoperative determination ofMVA was obtained 10-14 years (mean 12.2 years) after commissurotomy. In 17 patients, the MVA was determined by cross-sectional echocardiography and in one patient by repeat cardiac catheterization. Thirteen of 18 patients had no change in MVA between early postoperative study (mean MVA = 2.7 cm2) and late postoperative study (mean MVA = 2.9 cm2). MVA in five patients decreased 0.7-2.2 cm2 (mean 1.4 cm2) during the followup period. In these five patients, the mean MVA at early postoperative study was 2.7 cm2 and at late postoperative study was 1.3 cm2 (p < 0.001). At late postoperative evaluation, cardiac symptoms were associated with severity of mitral stenosis but did not predict restenosis. A successful, closed, instrumental mitral commissurotomy can provide substantial long-term improvement in MVA.