The 2005 Society for Hematopathology/European Association for Haematopathology Workshop session 11 was dedicated to hepatosplenic T-cell lymphoma (HSTCL). HSTCL is a rare aggressive type of extranodal lymphoma characterized by hepatosplenomegaly, bone marrow involvement, and peripheral blood cytopenias. HSTCL exhibits a distinctive pattern of infiltration; tumor cells preferentially infiltrate the sinusoids of the splenic red pulp, liver, and bone marrow. The tumor cells have a nonactivated cytotoxic T-cell immunophenotype and frequently carry a recurrent cytogenetic abnormality, isochromosome 7q. Most cases express the gammadelta T-cell receptor, but cases can have an alphabeta phenotype and are considered to be a variant of the disease. Although HSTCL is the prototype peripheral T-cell lymphoma expressing the gammadelta T-cell receptor, non-HSTCL proliferations of gammadelta T cells can involve other extranodal sites, mainly skin and mucosa. These gammadelta T-cell lymphomas display marked heterogeneity in clinical and histologic features. In contrast with HSTCL, non-HSTCL gammadelta T-cell lymphomas frequently have an activated cytotoxic phenotype and most likely are not a single disease entity.