Between 1985 and 1992, 36 consecutive neonates, aged 1-29 days, weight 2.4-5.0 kg, with critical valvar pulmonary stenosis underwent attempted balloon dilation (BD). At catheterization, 30 were on prostaglandin (PGE1) therapy and 20 were intubated. The valve was successfully crossed and dilated in 34/36 (94%), including three with an echocardiographic diagnosis of valvar pulmonary atresia and a right ventricle of adequate size. The valve was first dilated with a 2- to 5-mm balloon and then with serially larger ones (up to 12 mm) to a final balloon/annulus value of 126%. The RV/systemic pressure value fell from 150 +/- 32 to 83 +/- 30%, O2 saturation rose from 91 +/- 6% to 96 +/- 4%, and PGE1 was discontinued at the end of the procedure. There were 11 complications (31%) including one early death from sepsis and necrotizing enterocolitis, endocarditis in another, two myocardial perforations, one femoral-iliac vein tear, and one transient pulse loss. A repeat BD was carried out in five patients, two of whom subsequently had surgery. At follow-up (33 +/- 23 months), the 31 patients managed by BD alone were well and had echocardiographic gradients of < 30 mm Hg in 90% and pulmonary regurgitation, considered mild in most, in 52%. In neonates with critical valvar pulmonary stenosis, we believe BD mortality is less than with surgery and is the treatment of choice.