In seven patients with Duchenne dystrophy, high-resolution phase microscopy demonstrated a population on non-necrotic fibers with one or more focal lesions. The typical lesion was wedge-shaped, with the base resting on the fiber surface. In the electron microscope, the plasma membrane overlying the lesion was either absent or disrupted, while the basement membrane was always preserved. Within the lesion, there were cytoplasmic abnormalities, and in the neighboring fiber region, the myofibrils were usually highly contracted. The structural defect in the plasma membrane suggested that this site was an ineffective cellular barrier. This was confirmed by the frequent ingress of peroxidase-containing extracellular fluid into the lesions. In two control subjects, peroxidase penetration into fibers was seen only rarely and only with other evidence of mechanical injury to the specimen. The findings point to an early and possibly basic abnormality in the plasma membrane of the muscle fiber in Duchenne's dystrophy.