Electron microscopic studies revealed distinctive abnormalities in operatively resected myocardium from the left ventricular outflow tract in 14 patients with idiopathic hypertrophic subaortic stenosis. Bundles of muscle cells were severely disorganized, with cells running in different directions instead of in parallel. Muscle cells were wider and shorter than in hypertrophy due to other causes and showed increased cellular branching, extensive side-to-side intercellular junctions, widened Z bands, and evidence of formation of new sarcomeres. Some myofibrils were oriented obliquely or perpendicular to the longitudinal axes of the cells and some myofilaments that originated from a single Z band inserted into Z bands of other myofibrils. Examination of left ventricular apical myocardium in two patients revealed hypertrophied but normally arranged muscle cells. It is concluded that abnormal architecture of muscle cells is the basic morphologic feature of idiopathic hypertrophic subaortic stenosis.