OBJECTIVE--To assess the outcome after attempted percutaneous balloon dilatation of the mitral valve in patients with severe mitral stenosis between February 1986 and June 1992. DESIGN--Clinical state, mitral valve area, and restenosis at follow up were analysed. Mitral valve area as determined by the Gorlin formula, planimetry, and Doppler methods was compared before and after dilatation and at follow up. SETTING--University hospital. PATIENTS--176 patients had serial clinical and Doppler echocardiographic follow up and 44 of them also underwent recatheterisation. RESULT--At follow up 93% of patients were in New York Heart Association functional class I or class II. Mitral valve area (planimetry) increased from 0.97(0.24) cm2 before to 1.86(0.39) cm2 after dilatation (p = 0.0001) and then decreased to 1.72(0.39) cm2 at follow up (p < 0.001); mitral valve area (Doppler) increased from 1.01 (0.24) to 1.89 (0.42) cm2 (p = 0.0001) and then decreased to 1.78(0.40) cm2 (p < 0.05). The overall restenosis rate was 15% and over 90% of the patients were free from cardiovascular events. Age, valvar calcification, echocardiographic score, and mitral valve area after dilatation were found to be determinant predictors of restenosis. In patients who underwent recatheterisation, mitral valve area by the Gorlin method at follow up was comparable with that by planimetry and Doppler methods whereas a significant discrepancy between the three methods was noted immediately after dilatation. CONCLUSION--Balloon dilatation of the mitral valve provided sustained anatomical and functional improvement in over 80% of patients at late follow up. Older age, heavy calcification, high echocardiographic score, and suboptimal immediate results are significant predictors of restenosis. Doppler echocardiographic examination is the procedure of choice for follow up evaluation.