Abstract： Background Rhabdomyosarcoma (RMS) is an uncommon malignancy of the breast.The aim of this study was to summarize its clinicopathologic features and biological behavior.Methods Five primary or secondary breast RMSs were collected.Their clinicopathological characteristics and all published literature about breast RMS were reviewed.Immunohistochemical study of desmin,myogenic differentiation 1 (MyoD1),myogenin,leukocyte common antigen (LCA),vimentin,cytokeratin (AE1/AE3),E-cadherin,neuron specific enolase (NSE),CD99,chorioallantoic membrane 5.2 (CAM5.2) and epithelial membrane antigen (EMA) expression were performed.Results The five patients were all female with ages ranging from 16 to 46 years old (mean,30 years).Three were metastatic breast RMSs,two embryonal and one solid variant alveolar,with the primary tumor sites the right labium majus,left nasal meatus and nasopharynx,respectively.The other two,one embryonal and one alveolar,were primaries.Grossly,the surgical specimens revealed round or oval,well-demarcated but nonencapsulated masses.Their cut surfaces consisted of homogeneous grayish yellow or white tissue.Microscopically,most tumor cells were poorly differentiated small round,oval or small polygons with eosinophilic cytoplasm.All cases were positive for vimentin,desmin,MyoD1 and myogenin.One embryonal RMS also had a few cells with perinuclear staining of AEI/AE3.The other markers were negative.Conclusions Although primary or metastatic RMS in breast was almost confined to young adolescent females,our cases suggested that it can also happen to the middle-aged women.Embryonal RMS has a certain metastatic potential.MyoD1 and myogenin are two useful markers when making differential diagnosis.Axillary lymph node status and age may play a role in the prognosis of primary breast RMS patients.