Aim: To determine (a) the expression of plasma cell related antigens in extran odal marginal zone B cell lymphomas (EMZL) of the ocular adnexa; and (b) the pro gnostic value of plasmacellular differentiation in these tumours. Methods: A con secutive case series of 136 ocular adnexal EMZL obtained from three ocular patho logy centres over 20 years was analysed retrospectively. An extensive immunohist ochemical panel, including the plasma cell related antigens VS38c, CD38, CD138, multiple myeloma oncogene-1-protein (MUM1/ IRF4), and CREB binding protein (CB P) was performed. EMZL were defined as “plasmacellular differentiated”on the b asis of morphological features, evidence of cytoplasmic immunoglobulin, negativi ty for BSAP/PAX5, and expression of at least one of the investigated plasma cell related antigens. Controls included normal or hyperplastic lymphatic tissues. D etailed clinical data were collected for most patients, and compared with the re sults of immunohistochemistry. The end points considered for statistical analysi s were development of local tumour recurrence, development of systemic disease, and lymphoma related death. Results: 57 (42%) of the 136 ocular adnexal EMZL sh owed a plasmacellular differentiation; 45 of these plasmacytoid cases were prima ry tumours. In contrast with most admixed normal plasma cells, which displayed c o-expression of MUM1/IRF4, Vs38c, CD38, CD138, and CBP, the plasmacellular diff erentiated EMZL tumour cells demonstrated co-expression of all five plasma cell related antigens in only six of 57 (11%) plasmacellular differentiated ocular adnexal EMZL. The most commonly expressed plasma cell related antigen was MUM1/I RF4, immunoreactivity being seen in 56/57 (98%) plasmacellular differentiated E MZL examined. Although the association of plasmacellular differentiation in prim ary ocular adnexal EMZL and disseminated disease was statistically significant o n univariate analysis (p=0.042), this was weaker on multivariate analysis. Concl usion: Plasmacellular differentiated tumour cells in EMZL demonstrate an aberran t immune profile for plasma cell related antigens when compared with normal plas ma cells. On multivariate analysis, plasmacellular differentiation in ocular adn exal EMZL was not significantly associated with local recurrence, the developmen t of systemic disease, or with lymphoma related death. Aim: To determine (a) the expression of plasma cell related antigens in extran odal marginal zone B cell lymphomas (EMZL) of the ocular adnexa; and (b) the pro gnostic value of plasmacellular differentiation in these tours. Methods: A con secutive case series of 136 ocular adnexal EMZL obtained from three ocular patho logy centers over 20 years was identi- fied retrospectively. An extensive immunohistochemical panel, including the plasma cell related antigens VS38c, CD38, CD138, multiple myeloma oncogene-1-protein (MUM1 / IRF4 ), and CREB binding protein (CB P) was performed. EMZL were defined as “plasmacellular differentiated” on the b of the morphological features, evidence of cytoplasmic immunoglobulin, negativi ty for BSAP / PAX5, and expression of at least one of the investigated Plasma included related antigens. Controls included normal or hyperplastic lymphatic tissues. D etailed clinical data were collected for most patients, and compared with the re sults of immunohistochemistry. T Results of 57 (42%) of the 136 ocular adnexal EMZL sh owed a plasmacellular differentiation; 45 of these plasmacytoid cases were prima ry tumours. In contrast with most admixed normal plasma cells, which displayed c o-expression of MUM1 / IRF4, Vs38c, CD38, CD138, and CBP, the plasmacellular diff erentiated EMZL tumor cells demonstrated co-expression of all five plasma cells related Antigens in only six of 57 (11%) plasmacellular differentiated ocular adnexal EMZL. Mostly expressed plasma cell related antigen was MUM1 / I RF4, immunoreactivity being seen in 56/57 (98%) plasmacellular differentiated E MZL examined. of plasmacellular differentiation in prim ary ocular adnexal EMZL and disseminated disease was significantly significant on univariate analysis (p = 0.042), this was weaker on multivariate ana lysis. Concl usion: Plasmacellular differentiated tumor cells in EMZL demonstrate an aberran t immune profile for plasma cell related antigens when compared with normal plas ma cells. On multivariate analysis, plasmacellular differentiation in ocular adn exal EMZL was not significantly associated with local recurrence, the developmen t of systemic disease, or with lymphoma related death.