The tumor necrosis factor (TNF) receptor family includes several important markers of activation in T cells. We examined expression patterns of two T-cell-associated members of these receptors, namely CD30 and OX40/CD134, in 148 cases of T-cell lymphoma to identify possible objective immunohistochemical criteria for subclassification of these tumors. CD30 expression was characteristic of tumors with an anaplastic (46/47 cases [98%]) or large-cell (10/21 [48%]) morphology and was seen in only scattered cells in other tumor types. In contrast, large numbers of OX40/CD134(+) tumors cells were typical of angioimmunoblastic lymphoma (15/16 [94%]), angiocentric lymphoma (4/4), a subset of large-cell lymphomas (10/21 [48%]), and lymphomas with a prominent histiocytic component (6/7 [86%]). Strong OX40/CD134 and CD30 coexpression was seen in only 4% of tumors, typically those with an anaplastic/Hodgkin's-like appearance. OX40/CD134 expression was characteristic of tumors composed of activated CD4(+) T cells and was not seen in small-cell T-cell lymphomas, lymphoblastic lymphomas, or other tumor types, including B-cell lymphomas or carcinomas. These results suggest that immunostaining for OX40/CD134 may be helpful in subclassification of peripheral T-cell lymphomas and that the patterns of TNF receptor family expression in these tumors may parallel those seen within nonneoplastic helper T-cell subsets.