Angioimmunoblastic T-cell lymphoma (T-AIL) is a peripheral T-cell lymphoma of unknown etiology. Previous clonality studies have shown a heterogeneous composition of this disease with varying restrictions of B- and T-cell populations in the tumour. For the first time in a single study and in the same pathological materials, we have analysed, lymphoid cell clonality and occurrence of human herpes viruses and Epstein Barr virus. Of 18 cases 12 (66.6%) had clonal T- and three (16.6%) had clonal B-cells. Presence of the lymphotropic viral genome of HHV6 was detected in four of 18 lymph node biopsies from T-AIL patients (22%), all were TCRγ clonal. No HHV8 were found. Epstein Barr genome was found in 40% of cases. There was no significant association between T-cell clonality and HHV-6 or EBV infection, or between B-cell clonality and any virus infection. We conclude that T-AIL is a biologically and clinically heterogeneous entity whose true nature remains to be clarified. Copyright © 2005 John Wiley & Sons, Ltd.