Angioimmunoblastic T-cell lymphoma (AITL) is an uncommon, but aggressive nodal peripheral T-cell lymphoma. Little is known of its biology and its natural history has been poorly studied. We report the first comprehensive study on the natural history/histologic progression of AITL by reviewing consecutive biopsies in 31 cases. Immunostaining for CD3, CD20, CD10 and CD21, CD23, CNA-42, CD4, CD8, and Ki 67, in situ hybridization for Epstein-Barr virus (EBV)-encoded RNA and polymerase chain reaction for T-clonality and B-clonality were performed. Histologic progression from AITL with limited nodal involvement and hyperplastic follicles (pattern I) to typical AITL with or without regressed follicles (patterns II and III) was observed in 7 cases, one of which relapsed subsequently as pattern I. Thirteen cases showed typical AITL at presentation and follow-up. Eleven cases where polymerase chain reaction results for T-cell receptor-gamma gene rearrangement were directly compared showed an identical band-size in the initial and follow-up biopsies. Seven cases (23%) developed EBV-associated B-cell lymphomas [5 diffuse large B-cell lymphoma (DLBCL) and 2 classic Hodgkin lymphoma]. In 4 cases, a dominant B-cell clone was observed in biopsies lacking evidence of DLBCL. A single case was complicated by EBV-negative DLBCL, whereas another with large cell transformation had a T-cell phenotype. In conclusion, AITL represents a clonal T-cell proliferation with a stable T-cell clone throughout the disease. Partial nodal involvement with hyperplastic follicles is seen in early AITL and at relapse. When "morphologic high-grade transformation" occurs, it is usually due to a secondary (often EBV-associated) B-cell lymphoma, rather than a T-cell neoplasm.