While present in the surface membrane of embryonic muscle fibers, in adult normal muscle fibers, utrophin is restricted to the motor endplate and cells of blood vessel walls. However, the observation that utrophin is maintained in the extrajunctional plasma membrane in Duchenne (DMD) and in mdx muscle fibers has led to the suggestion that excess utrophin might compensate for dystrophin deficiency in the Xp21 muscular dystrophies. In order to detect an inverse correlation of utrophin presence and clinical severity, we have assessed utrophin distribution and quantity in DMD and Becker (BMD) patients of different ages and stages of clinical severity. All patients showed a positive discontinuous immunolabeling of utrophin on the sarcolemma, staining equally small and large muscle fibers, indicating that immature characteristics are maintained in such fibers. On Western blot, utrophin bands with concentrations 2- to 10-fold greater than in normal controls were detected in all DMD/BMD patients. However, no negative correlation was found between the amount of utrophin and the severity of clinical course, implying that the detectable utrophin levels in these patients did not compensate for dystrophin deficiency. In a DMD patient with growth hormone (GH) deficiency and a BMD-like clinical course, utrophin levels were comparable to the other typical DMD cases, which reinforces the hypothesis that the observed increase in utrophin is apparently not responsible for a milder clinical course in some patients with Xp21 muscular dystrophies.