Dystrophin is a protein product of the Xp 21 gene which is defective in patients with Duchenne muscular dystrophy (DMD). In immunohistochemical staining of dystrophin, the majority of DMD muscle fibers show negative staining. Nevertheless, biopsy specimens from DMD patients labeled with many different antibodies may show a small number of fibers which are clearly dystrophin positive. This very small percentage of dystrophin-positive fibers (DPF) probably represents somatic reversion, suppression of the DMD gene mutation or alternating splicing of dystrophin mRNA. To determine the significance of isolated DPF in muscle specimens of DMD patients, we examined 30 DMD muscle specimens, aged 4 years to 16 years, by the use of monoclonal antibody against the C-terminal region of dystrophin. Additionally, muscle specimens from 2 normal human controls and 2 mice with X-linked muscular dystrophy were used for positive and negative controls, respectively. Muscle specimens from DMD patients and mdx mice showed almost totally negative dystrophin staining in most of their muscle fibers, but in 20 patients, three was a trace of isolated DPF ranging from 0.06% to 0.77%. DMD patients with no isolated DPF seemed to have higher functional disability. In the whole group of 30 patients, a significant negative correlation was found between the abundance of DPF and clinical functional grading (r = -0.85, p < 0.0001, based on linear regression). It is suggested that even the very low concentrations of dystrophin found in DMD patients may have a favorable functional significance.