Percutaneous balloon dilation of the aortic valve has recently been proposed as a palliative procedure for treating nonsurgical candidates with calcific aortic stenosis. To assess the safety, efficacy and mechanisms of successful balloon valvuloplasty, postmortem (n = 33) and intraoperative (n = 6) balloon aortic valvuloplasty was performed in the hearts of 39 elderly patients with calcific aortic stenosis. The cause of aortic stenosis was degenerative nodular calcification in 28 cases, calcific bicuspid aortic stenosis in 8 cases and rheumatic heart disease in 3 cases. Balloon dilation was performed with 15 to 25 mm balloons in the postmortem specimens, and with 18 to 20 mm balloons in the operating room immediately before aortic valve replacement. After balloon dilation, valve orifice dimensions and leaflet mobility increased in all patients. The mechanisms of successful dilation included fracture of calcified nodules in 16 aortic valves, separation of fused commissures in 5 valves, both in 6 valves and grossly inapparent microfractures in 12 valves. Valve leaflet avulsion occurred in one heart after inflation with a clearly oversized balloon. Liberation of calcific debris, valve ring disruption or midleaflet tears did not occur in any heart. In conclusion, there are at least three mechanisms of successful aortic valvuloplasty, depending on the origin of valvular stenosis. Embolic phenomena and acute valvular regurgitation do not appear to be likely events associated with this procedure.