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To describe the clinicopathological and immunophenotypic features of 26 cases of extra-medullary myeloid tumour (EMMT)/granulocytic sarcoma, which remains poorly recognized and is frequently confused with malignant lymphoma, and to discuss the main diagnostic problems experienced by the referring pathologist.Haematoxylin and eosin (H & E) sections of 26 cases of EMMT were re-examined. Immunostains for myeloperoxidase, lysozyme, neutrophil elastase, LCA, CD79a, CD20, CD43, CD45RO, CD3, CD30, CD15, CD68, MAC387, VS38C, MIC2, and the Leder stain for naphthol-ASD-chloroacetate esterase were performed on all cases. Clinical and follow-up data were obtained through a questionnaire to the referring pathologist or from the notes of the patients where available. In the 10 cases with known myeloproliferative disease, the initial diagnosis was correct in 10 whereas all cases presenting with EMMT without a previous history of myeloproliferative disorder had an initial incorrect diagnosis. The most common suggested diagnosis was that of a non-Hodgkin's lymphoma. The morphology of the tumours varied from well differentiated which included all stages of myeloid differentiation to poorly differentiated or blastic showing little or no evidence of myeloid differentiation. The proportion of positive cells for each stain varied. Chloroacetate esterase, myeloperoxidase and CD15 stained a large proportion of cells of the majority of the well differentiated tumours and a smaller proportion of the poorly differentiated/blastic tumours with very focal staining of some of the cases. Lysozyme and CD43 were the most sensitive of the markers staining a large proportion of cells of the majority of the tumours in both groups. Neutrophil elastase was the least sensitive of the markers of myeloid differentiation. CD79a, CD20, CD3 and CD30 were negative in all cases. CD43 was positive in all cases. CD68 stained a substantial number of cells in the majority of tumours. A smaller proportion of the tumours stained with MAC387. Four of the tumours showed positivity for MIC2. One tumour was positive for VS38C.This series documents continuing difficulties in the diagnosis of EMMT. Even well differentiated tumours are frequently mistakenly diagnosed as malignant lymphomas when they present without any history of antecedent myeloproliferative disorder. Careful evaluation of morphology for evidence of myeloid differentiation and a high index of suspicion when confronted with a less differentiated neoplasm are required to avoid this important diagnostic error. We suggest that a panel which includes chloroacetate-esterase, myeloperoxidase, lysozyme and CD43, together with other B- and T-lineage markers, in particular CD79a and CD3 should be used to confirm the diagnosis.