Retrograde catheterization of the left atrium utilizing the Shirley technique was successfully performed in four patients with idiopathic hypertrophic subaortic stenosis. By traversing the inflow tract of the left ventricle on pullback from the left atrium to the aorta, this method served to separate definitively true hypertrophic subaortic stenosis from cavity obliteration. It was accomplished with relative ease and no complications. In patients with suspected idiopathic hypertrophic subaortic stenosis who are to have retrograde left heart catheterization, this approach to the left atrium is recommended as an alternative to the transseptal technique.