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We present the case of a 16-year-old boy admitted with a large mediastinal mass. Both in the mediastinal tru-cut biopsy and the bone marrow biopsy, a malignant small round cell tumor was determined. The clinical diagnosis was leukemia or lymphoma. The cytoplasmic CD20 expression was shown on the tumor cells of the mediastinal tru-cut biopsy. Rhabdomyosarcoma was diagnosed by the flow cytometric analysis and immunohistochemical demonstration of muscle markers. Rhabdomyosarcomas are traditionally distinguished from lymphomas by their absence of lymphoid markers such as CD20. But in this case we have encountered CD20 expression in the tumor cells. This finding lead us to conclude that the presence of a lymphoid phenotype does not absolutely exclude the diagnosis of rhabdomyosarcoma.