The 2005 Society for Hematopathology/European Association for Haematopathology Workshop Session 1 was devoted to case presentations with discussions of 3 types of mature T-cell leukemias--T-cell prolymphocytic leukemia, adult T-cell leukemia/lymphoma, and Sézary syndrome. These 3 disorders are clonal proliferations of postthymic alphabeta T cells that are often characterized by systemic manifestations and a leukemic blood picture. The application of clinical, morphologic, immunophenotypic, and genetic studies to the assessment and characterization of these 3 disorders is presented, along with specific diagnostic recommendations and differential diagnostic considerations.