The cytokine leukemia inhibitory factor (LIF)stimulates myoblast proliferationin vitro and vivoand is neurotrophic for motor neurons. In experimentally reinnervated muscle, exogenous. LIF application increases muscle mass through myofiber hypertrophy. The goal ofthis study was to evaluate possible sources of endogenous LIF in human muscle, and whether LIF immunoreactivity (-IR) was detectable in specific myofiber types and/or re-expressed in human denervated muscle. Our study shows that LIF-IR is constitutively detectable in type I myofibers of normal human muscle. In acute and chronically denervated and reinnervated human muscle, LIF-IR is found in all type I myofibers and in addition in some atrophic and almost all angulated atrophic type II myofibers.