Primary mediastinal large B-cell lymphoma is a well-defined lymphoma entity whose molecular pathogenesis is incompletely understood and also lacking well-established diagnostic markers. Recently, the presence of overlapping features between classical Hodgkin's lymphoma and primary mediastinal large B-cell lymphoma was highlighted by gene expression profiling as well as morphological studies. We investigated the expression of TP73L (commonly known as p63) isoforms in primary mediastinal large B-cell lymphoma at both protein and mRNA level, and demonstrated the exclusive presence of transactivating (TA) isoforms in all cases. We also demonstrated that TP73L is expressed in a subset of germinal center B-cells, as well as in some diffuse large B-cell lymphomas, but it is never present in classical Hodgkin lymphoma. Nodular lymphocyte predominant Hodgkin's lymphoma also showed TP73L positivity by immunohistochemistry. Isoform analysis by real-time PCR showed that TA-TP73Lalpha is the most represented in primary mediastinal large B-cell lymphoma, but TA-TP73Lgamma is the most differentially expressed in comparison to both germinal center B-cells and diffuse large B-cell lymphomas. TP73L expression proved a useful diagnostic marker of primary mediastinal large B-cell lymphoma, and gave new insights in to the molecular pathways playing a role in this lymphoma.