Non-Hodgkin's B cell lymphoma (NHL) is a common cancer in HIV infection. Many NHL are thought to result from errors in class switch recombination and/or somatic hypermutation, processes that occur in germinal center B cells, and require the activity of activation induced cytidine deaminase (AID). Since NHL is a common cancer in HIV infection, and expression of AID could contribute to the development of NHL, we hypothesized that AID expression would be elevated in those who went on to develop AIDS-associated NHL (AIDS-NHL). AID mRNA levels were measured by TaqMan RT-PCR in peripheral blood mononuclear cells, obtained prior to AIDS-NHL diagnosis, from 16 HIV-infected subjects who developed AIDS-NHL, and from control subjects (AIDS but no NHL, and HIV-negative subjects). PBMC AID expression was markedly elevated in those who developed AIDS-NHL, when compared to AIDS and HIV-negative controls. Additionally, AID expression was seen to differ depending on NHL subtype, with the highest levels of expression seen in those who developed Burkitt's lymphoma.