In this study, we have demonstrated that Activin A induces myofibroblast differentiation of endometrial MSCs via STAT3-dependent Smad/CTGF pathway and that blocking Activin A signal impedes myofibroblast differentiation of endometrial MSCs in vitro and ameliorates fibrosis in endometriosis in a mouse model. This evidence concerns the gene STAT3 and endometriosis.