AimsMucin 4 (MUC4) is a transmembrane glycoprotein normally expressed by several human epithelial surfaces, including those of the colon, vagina, and respiratory tract. Although MUC4 overexpression is seen in various carcinomas, its expression among mesenchymal neoplasms is fairly specific to low‐grade fibromyxoid sarcoma and sclerosing epithelioid fibrosarcoma. Having observed unanticipated anti‐MUC4 immunoreactivity in rhabdomyosarcoma, we aimed to further characterise its expression.Methods and resultsExpression of MUC4 was assessed by immunohistochemistry in a total of 97 rhabdomyosarcomas using formalin‐fixed paraffin‐embedded tissue sections. MUC4 was expressed by 21 of 26 PAX3/7–FOXO1 fusion‐positive cases, wherein immunoreactivity, varying from weak to strong, was present in 20–100% of neoplastic cells. With the exception of one sclerosing rhabdomyosarcoma showing immunoreactivity in 20% of cells, MUC4 was not expressed by embryonal (n = 28), sclerosing (n = 20), or pleomorphic (n = 23) rhabdomyosarcomas. Analysing published gene expression microarray data from a separate cohort of 33 fusion‐positive and 25 fusion‐negative rhabdomyosarcomas, we found on average a 11.4‐fold increased expression in fusion‐positive tumours (P = 0.0004).ConclusionsMUC4 is expressed to a variable extent in the majority of PAX3/7–FOXO1 fusion‐positive (alveolar) rhabdomyosarcomas, while expression in other rhabdomyosarcoma subtypes is rare.