A lthough the high incidence of EBV-associated diffuse large cell lymphomas (DLCL) in HIV-1 infection is believed to be related to loss of immune control due to HIV-induced immune deficiency, it has been claimed that cytotoxic T lymphocyte (CTL) responses to EBV are longer lasting in HIV-1-infected persons than CTL directed against HIV-1 itself. We approached this apparent paradox by performing the first longitudinal study into the kinetics of EBV and HIV-specific CTL responses in HIV-infected patients progressing either to AIDS with non-Hodgkin's lymphoma (NHL) or AIDS with opportunistic infection (OI). Multiple samples were tested from HIV-1 seroconversion to AIDS-diagnosis. Four out of six patients that were either long-term asymptomatic or progressing to OI showed declining HIV-1 CTL precursor (CTLp) frequencies whereas EBV-CTLp remained stable, suggestive for HIV-1-specific immune exhaustion. In two patients rapidly progressing to AIDS-OI, a parallel decline of HIV-1- and EBV-CTL responses was seen, indicative for total collapse of cellular immunity. In all these six patients EBV-load remained low. However, in four out of five patients that progressed to DLCL, EBV-load was high and increasing several months preceding the NHL. In all five patients, EBV-CTLp decreased before the emergence of the NHL. Thus, our data show that in HIV-1 infection loss of HIV-1-specific T cell immunity is not necessarily paralleled by loss of EBV-specific T cell responses. The occurrence of AIDS-related DLCL is preceded by decreasing EBV-CTLp and increasing EBV load. Failing EBV-control might therefore be an important step in the pathogenesis of AIDS-related DLCL.