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Background T follicular helper (TFH) cells reside in the light zone of germinal centers and are considered the cell of origin of angioimmunoblastic T-cell lymphoma. Recently, CXCL13, PD-1 and SAP were described as useful markers for TFH cells and angioimmunoblastic T-cell lymphoma but also reported in some peripheral T-cell lymphomas, not otherwise specified.Design and Methods In the present study the expression pattern of ICOS protein was investigated by immunohistochemistry-based techniques in routine sections of normal lymphoid tissues and 633 human lymphomas.Results Cells strongly positive for ICOS were restricted to the light zone of germinal centers and co-expressed TFH-associated molecules. In addition, weak to moderate ICOS expression was observed in a small proportion of FOXP3-positive cells. In lymphomas, ICOS expression was confined to angioimmunoblastic T-cell lymphoma (85/86), peripheral T-cell lymphomas of follicular variant (18/18) and a proportion of peripheral T-cell lymphomas, not otherwise specified (24/56) that also expressed other TFH-associated molecules.Conclusions ICOS is a useful molecule for identifying TFH cells and its restricted expression to angioimmunoblastic T-cell lymphoma and a proportion of peripheral T-cell lymphomas, not otherwise specified (showing a TFH-like profile) suggests its inclusion in the antibody panel for diagnosing TFH-derived lymphomas. Our findings provide further evidence that the histological spectrum of TFH-derived lymphomas is broader than previously assumed.