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Purpose Peripheral T-cell lymphoma (PTCL) and natural killer/T-cell lymphoma (NKTCL) are rare and heterogeneous forms of non-Hodgkin's lymphoma (NHL) that, in general, are associated with a poor clinical outcome. Patients and Methods A cohort of 1,314 cases of PTCL and NKTCL was organized from 22 centers worldwide, consisting of patients with previously untreated PTCL or NKTCL who were diagnosed between 1990 and 2002. Tissue biopsies, immunophenotypic markers, molecular genetic studies, and clinical information from consecutive patients at each site were reviewed by panels of four expert hematopathologists and classified according to the WHO classification. Results A diagnosis of PTCL or NKTCL was confirmed in 1,153 (87.8%) of the cases. The most common subtypes were PTCL not otherwise specified (NOS; 25.9%), angioimmunoblastic type (18.5%), NKTCL (10.4%), and adult T-cell leukemia/lymphoma (ATLL; 9.6%). Misclassification occurred in 10.4% of the cases including Hodgkin's lymphoma (3%), B-cell lymphoma (1.4%), unclassifiable lymphoma (2.8%), or a diagnosis other than lymphoma (2.3%). We found marked variation in the frequency of the various subtypes by geographic region. The use of an anthracycline-containing regimen was not associated with an improved outcome in PTCL-NOS or angioimmunoblastic type, but was associated with an improved outcome in anaplastic large-cell lymphoma, ALK positive. Conclusion The WHO classification is useful for defining subtypes of PTCL and NKTCL. However, expert hematopathology review is important for accurate diagnosis. The clinical outcome for patients with most of these lymphoma subtypes is poor with standard therapies, and novel agents and new modalities are needed to improve survival.