Mitral lesion in 47 patients who received reoperation after an initial open mitral commissurotomy (OMC) for mitral stenosis (MS) was studied. The diagnosis before OMC consisted of pure MS in 38 patients, with mild mitral regurgitation (MR) in 8 and with moderate MR in 1. At OMC the diameter of the mitral orifice was a mean of 1.3 +/- 0.5 cm and a mobile cusp was found in 12 and a thickened cusp in 35 patients. Subvalvular fusion was seen in 15 patients, calcification of cusp or subvalvular region in 10 and left atrial thrombus in 6. After OMC, the diameter of the mitral orifice was opened to a mean of 3.2 +/- 0.3 cm. The interval between the initial operation and the time of reoperation was a mean of 124 months. The mitral lesion necessitating reoperation was MS in 18, moderate to severe MR in 10, and MSR in 19. At reoperation the mitral orifice ranged in diameter from 0.8 to 4.2 cm (mean, 2.3 +/- 0.8 cm). There was no correlation between the occurrence of MR or restenosis and the subvalvular division at initial OMC. Left atrial thrombus was identified in four and calcification in 17 patients. At reoperation 46 patients received mitral valve replacement, while only 1 received a second OMC. The removed valve showed no pathological findings of active rheumatic lesion but more severe calcification and hyalinization of the valve than those at initial operation. There were three early deaths (6.4%) after reoperation. The recurring mitral lesion seemed to consist of deteriorated pathological changes of the valve and MR and thus mitral valve replacement was recommended to provide a favorable operative result for non-flexible lesion after OMC.