In the World Health Organization (WHO) 2008 classification 2 main types of primary cutaneous large B-cell lymphomas (PCLBCLs) are distinguished: primary cutaneous follicle center lymphoma (PCFCL) and primary cutaneous large B-cell lymphoma, leg type (PCBCL-LT). PCFCL has a 5-year overall survival rate of 95%, and PCBCL-LT of approximately 50%. Expression profiling studies have shown higher RNA expression of the IgM heavy chain in PCBCL-LT compared with PCFCL. To find out whether this difference could also be demonstrated at the protein level, we performed immunohistochemical staining for B-cell receptor heavy and light chains on skin biopsies from 53 patients with PCFCL and 40 patients with PCBCL-LT. All 40 cases of PCBCL-LT consistently showed cytoplasmic staining for IgM, in 18 of them with coexpression of IgD. In contrast, only 5 of the PCFCL cases showed cytoplasmic staining for IgM and/or IgD, including all 3 PCFCLs presenting on the leg. Hence, staining for IgM on paraffin-embedded sections seems to be an additional tool for differentiating between the 2 entities in clinical pathology practice. Analogous to other nodal and extranodal large B-cell lymphomas, expression of IgM in PCLBCL seems to be related to an activated B cell-like phenotype. Finally, the expression of IgM (and IgD) in this type of lymphoma might imply defective class switch recombination.