Percutaneous balloon mitral valvotomy has recently been developed as an alternative to surgical commissurotomy for patients with rheumatic mitral stenosis. We analyzed our initial experience with 60 consecutive procedures performed in 49 patients over 1 1/2 years and identified factors influencing the immediate hemodynamic results. For the total patient population, the mitral valve area increased after percutaneous mitral valvotomy from 0.8 +/- 0.04 to 1.6 +/- 0.11 cm2 (p less than 0.001). Mean diastolic mitral gradient fell from 18 +/- 1 to 7 +/- 0.4 mm Hg (p less than 0.001), and cardiac output increased from 3.8 +/- 0.2 to 4.5 +/- 0.2 L/min (p less than 0.01). Although percutaneous mitral valvotomy resulted in an increase in mitral valve area in each patient, a suboptimal result, as defined by a postprocedure mitral valve area of 1.0 cm2 or less, an increase in area of 25% or less, or a final mitral gradient of 10 mm Hg or more occurred in 21 of the 60 procedures (35%). Multivariate analysis of 16 variables was performed to determine which factors might predict this result. Patients with a suboptimal result were more likely to have severe valve leaflet thickening or immobility and an extreme degree of subvalvular thickening and calcification on echocardiogram. Other factors that predicted a suboptimal result were a smaller effective balloon dilating area and the presence of atrial fibrillation. Thus optimal immediate hemodynamic results can be obtained in the majority of patients undergoing percutaneous mitral valvotomy. Optimal results may be expected in patients in normal sinus rhythm, with pliable mitral leaflets, and with no severe subvalvular disease identified by echocardiography, who undergo dilation with large effective balloon dilating areas.