Percutaneous balloon valvotomy was attempted in 27 patients (aged 6 days to 19 years, median 2 years, 11 months) with unoperated typical valvular pulmonary stenosis using a balloon 7 to 60% (mean 30%) larger than the valve anulus. One patient had undergone a previous balloon valvotomy elsewhere. To achieve an oversized dilation diameter in three larger patients, two balloons were inflated side by side. Their "effective dilation diameter" was determined by the diameter of the circle with the same area as that of the oval enveloping the two balloons. A significant reduction of the transvalvular gradient occurred in all patients (mean +/- SD = 74.3 +/- 14.7%, range 33 to 100%). The average gradient of 65.0 +/- 19.0 mm Hg (mean +/- SD) fell to 15.9 +/- 7.6 mm Hg (0 to 30 mm Hg). Twenty-five of 27 patients had a residual transvalvular gradient of less than 25 mm Hg. The calculated valve orifice area increased by an average of 183 +/- 80%. No significant complications occurred. It is concluded that percutaneous balloon valvotomy with a balloon 20 to 40% larger than the valve anulus is the treatment of choice for typical congenital valvular pulmonary stenosis.