Background Despite current medical and surgical therapy, infants with symptomatic congenital mitral stenosis (CMS) continue to have high rates of morbidity and mortality.Catheter balloon dilation has been successful in relieving symptoms in a few older children with CMS but has not been evaluated in infants.30% in 15 of 18 initial attempts, from 20.3+-8.2 to 10.9+-4.9 mm Hg (P<.001), and the mitral valve area increased from 0.7+-0.3 to 1.0+-0.5 cm2/M2(n=10, P=.01). No infants died during the initial balloon dilation, although 2 of 3 died during a repeat procedure for restenosis. Other complications included significant mitral regurgitation in 7 of 18 patients (39%), 4 of whom had SVMR. Of the 18 infants, 8 (44%) had persistent symptomatic improvement at a mean follow-up of 14 months (range, 2 to 32 months). The 2-year survival after balloon dilation was 70%; 40% remained free of repeat intervention. Mitral valve surgery in 13 infants consisted of SVMR resections in 7, mitral valve replacements in 4, and LA-to-LV aortic valved homografts in 2. The operative mortality was 30%. Sustained improvement occurred in 8 (6 with SVMR) at 11 to 62 months of follow-up (mean, 30 months), with a 2-year survival of 60%.Conclusions Infants with severe CMS have 2-year mortality rates approaching 40% regardless of treatment modality. Balloon dilation significantly reduces the transmitral gradient in the majority, but symptomatic improvement persists in only 40%. Procedure-related mortality was associated with repeat balloon dilation in patients with left ventricular hypoplasia. Balloon dilation of "typical" CMS can provide symptomatic relief in many infants, allowing postponement of valve replacement, although infants with SVMR do better with surgical management. (Circulation. 1994;89:2099-2106.)