Percutaneous balloon pulmonary or aortic valvuloplasty was performed in 66 consecutive patients with no deaths. The transvalvular pressure gradient was reduced from 85 +/- 35 to 30 +/- 15 mm Hg (p less than 0.01) in 39 patients with congenital pulmonary valve stenosis and from 108 +/- 46 to 32 +/- 16 mm Hg (p less than 0.01) in 27 patients with congenital aortic valve stenosis. Subsequent mild aortic regurgitation occurred in seven patients and moderate regurgitation occurred in one patient. Operative evaluation of seven patients with pulmonary valve stenosis who had additional cardiac anomalies revealed the mechanisms of valve opening to be commissural splitting, cusp tear, or avulsion of the cusp from the anulus. Operative evaluation of two patients with residual high aortic valve gradients revealed minor degrees of commissural splitting. Although further evaluation is required to determine the long-term effects, early evaluation indicates that percutaneous balloon valvuloplasty may be useful in the definitive treatment of isolated pulmonary valve stenosis in some patients and of palliative value in others. The procedure is considered palliative in patients with aortic valve stenosis.