With the ability to accurately detect T cell lymphoid differentiation, both by immunophenotypic and genetic probe methods available only within the past decade, pathologists have become increasingly aware of similarities between some pleomorphic T cell lymphomas and Hodgkin's disease. In Workshop Session B, nine cases were reviewed in detail. Consensus was attained on seven cases, four of which were considered Hodgkin's disease and three of which were considered non-Hodgkin's lymphomas. Two of these seven cases exhibited morphologic features that alone would have indicated Hodgkin's disease, but manifested clinical, immunophenotypic, or genetic probe findings that instead favored their classification as non-Hodgkin's T cell lymphoma. In two cases, no consensus could be reached; ie, features in aggregate overlapped Hodgkin's disease and non-Hodgkin's lymphoma with T cell differentiation, and therefore their classification remained indeterminate. It is concluded that in most cases of differential diagnostic difficulty, consideration of the pathobiology and ancillary-study findings allows assignment to either Hodgkin's disease or (T cell) non-Hodgkin's lymphoma. However, in rare cases, even with exhaustive application of such criteria, this distinction cannot be made.