Left ventricular mid-cavitary obstruction after balloon dilation in isolated aortic valve stenosis in children. A hyperdynamic left ventricle can lead to post-dilation mid-cavitary obstruction in the absence of any preballoon mid-cavitary obstruction in children with aortic valve stenosis. The purpose of this study was to define the incidence and course of post-dilation mid-cavitary obstruction. We reviewed the catheterization and echocardiographic data of 35 patients, ages 3 days to 18 years (mean = 7.9 years), with isolated aortic valve stenosis who underwent balloon dilation with a fall in aortic valve gradient to less than 20 mm Hg. Post-dilation mid-cavitary obstruction was suspected from post-dilation left ventricular angiogram and was quantitated by echocardiographic and Doppler examinations performed immediately after dilation and in a 1-3 month follow-up period. Continuous wave Doppler directed by color flow Doppler was used for evaluation and differentiation of post-dilation mid-cavitary obstruction and residual aortic valve stenosis. Three children (3/35 [9%]) all under 2 years of age developed post-dilation mid-cavitary obstruction immediately after dilation. The obstruction within the cavity resolved spontaneously in all three patients. Conclusions: 1) Post-dilation left ventricular mid-cavitary obstruction can occur in children under 2 years of age after balloon dilation of severe aortic valve stenosis, and 2) total regression of post-dilation mid-cavitary obstruction occurs spontaneously in all patients. 