Twenty two children (age range 4 months-20 years) with pulmonary valve stenosis and ten children (age range 1 month-11 years) with coarctation of the aorta underwent balloon dilatation in the 29 month period between October 1983 and February 1986. Number 5-9 French catheters with 5-20 mm balloons were used according to the size of the angiographically measured pulmonary valve annulus or coarcted segment and the aorta proximal to coarctation. The peak inflation pressure used in the balloons varied from 2-5 atm (202-505 kN/m2) for pulmonary valve stenosis and 4-8 atm (404-808 kN/m2) for coarctation, and inflation lasted 8-15 s. At least four balloon dilatations were performed in every case. After balloon dilatation the mean (SD) peak systolic pressure gradient across the pulmonary valve fell from 98 (39) to 33 (13) mm Hg. The cardiac index did not change. After balloon dilatation of aortic coarctation the systolic pressure in the descending aorta rose from 86.4 (17.7) to 107.8 (20) mm Hg and the peak systolic pressure difference across the coarctation fell from 44.1 (19.1) to 8.5 (8.5) mm Hg. The diameter of the angiographically measured coarcted segment increased, the Doppler estimate of the pressure difference across the coarctation decreased, and the femoral pulses improved. There were no important complications. Long term follow up results for balloon dilatation of pulmonary valve stenosis were excellent. Similar follow up was not available for patients after dilatation of aortic coarctation. Percutaneous balloon dilatation for pulmonary stenosis and coarctation of the aorta is a safe and effective alternative to operation. It is the treatment of choice for pulmonary valve stenosis and it may become so for coarctation of the aorta in young children if long term results are favourable.