Purpose The International Peripheral T-Cell Lymphoma Project was undertaken to better understand the subtypes of T-cell and natural killer (NK) – cell lymphomas. Patients and Methods Angioimmunoblastic T-cell lymphoma (AITL) was diagnosed according to the 2001 WHO criteria by a central review process consisting of panels of expert hematopathologists. Clinical, pathologic, immunophenotyping, treatment, and survival data were correlated. Results Of 1,314 patients, 243 (18.5%) were diagnosed with AITL. At presentation, generalized lymphadenopathy was noted in 76% of patients, and 89% had stages III to IV disease. Skin rash was observed in 21% of patients. Hemolytic anemia and hypergammoglobulinemia occurred in 13% and 30% of patients, respectively. Five-year overall and failure-free survivals were 33% and 18%, respectively. At presentation, prognostic models were evaluated, including the standard International Prognostic Index, which comprised the following factors: age 60 years, stages III to IV disease, lactic dehydrogenase (LDH) normal, extranodal sites (ENSs) one, and performance status (PS) 2; the Prognostic Index for Peripheral T-Cell Lymphoma, comprising: age 60 years, PS 2, LDH normal, and bone marrow involvement; and the alternative Prognostic Index for AITL (PIAI), comprising: age 60 years, PS 2, ENSs one, B symptoms, and platelet count 150 109 /L. The simplified PIAI had a low-risk group (zero to one factors), with 5-year survival of 44%, and a high-risk group (two to five factors), with 5-year survival of 24% (P .0065). Conclusion AITL is a rare clinicopathologic entity characterized by an aggressive course and dismal outcome with current therapies.