The present study is an attempt to clarify the natural history of valvular and subvalvular aortic stenosis. Twenty-eight patients with isolated and uncomplicated congenital aortic stenosis who had serial hemodynamic evaluation without intervening surgery were selected. They were divided into three groups according to the severity of the peak systolic pressure gradient. At the initial study, 16 of the 28 patients had mild stenosis, 11 patients had moderate stenosis, and one patient had severe stenosis. On restudy, there were only 9 patients with mild stenosis, 12 patients with moderate stenosis, and 7 patients with severe stenosis. The cardiac index and right-sided hemodynamics were normal. The electrocardiographic and radiographic findings in relation to severity of the lesion were also observed. It is our feeling that both valvular and subvalvular aortic stenosis are progressive lesions, the subvalvular being more so than the valvular.