Dysferlin is a protein of the sarcolemma that is mutated in patients with limb girdle muscular dystrophy 2B, Miyoshi myopathy, and distal anterior myopathy. It has been implicated in muscle signaling and sarcolemma repair. To further understand its functional role we studied dysferlin expression in satellite cells (SCs) in normal and pathological human muscle biopsies, as well as in primary cultures of human skeletal muscle. Using immunohistochemistry we detected dysferlin-positive (Dysf+) SCs. Double staining with c-met+, a total SC marker, showed that the number of Dysf+ SCs ranged from 33.7% +/- 4.4% in normal muscle to 68.0% +/- 6.2% in pathological muscles, whereas double staining with MyoD/Dysf showed that all activated SC (MyoD+) were also Dysf+. These results indicate that dysferlin is upregulated in activated SCs. In vitro, immunohistochemistry, semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), and real-time PCR showed that both dysferlin mRNA and protein expression were higher in multinucleated myotubes than in the myoblast stage (p < 0.05). Furthermore, experiments of inhibition of myoblast fusion with amiloride, a type T calcium channel antagonist, showed that dysferlin levels were lower in treated than in non-treated cultures (p < 0.001), demonstrating that dysferlin expression reached peak levels upon differentiation into myotubes. These results and the in vivo findings of dysferlin expression when SCs are activated confirm the involvement of dysferlin in human muscle regeneration/repair and its possible role in fusion events during muscle development.