Muscle biopsies in 4 cases of Duchenne muscular dystrophy and two normal controls were examined by electron microscopy using lanthanum as a tracer for assessment of the T-system. Lanthanum was localized in the extracellular spaces, the basement membranes, intercellular spaces between muscle fibers and satellite cells, subsarcolemmal vesicles (caveola), motor end-plate, and the T-system in the controls. The T-system was sometimes connected with the caveolae, and was located at right angles to the myofibrils in the controls. In Duchenne muscular dystrophy, the T-system showed irregularities of position, tangle formations and dilatation of the diameters when observed using lanthanum as a tracer, even in muscle fibers which under light- and electron-microscopic examinations did not show any changes. Structure of the triad was destroyed and lanthanun was localized in the sarcoplasmic reticulum (SR). The changes of the T-system and triad seemed to play an important role in the high level of creatine phosphokinase (CPK) in the serum of Duchenne muscular dystrophy patients who do not show any clinical symptoms. Because of the destruction of the triad, CPK will easily flow out to the exterior of muscle fibers via the T-system.