Absolute CD4+ T cell counts and EBV-specific CD4+ T cell-dependent interferon-γ production were assessed in six HIV-positive individuals prior to development of PCNS lymphoma (“cases”), and these values were compared with those in 16 HIV-infected matched participants with no sign of EBV-associated pathology (“matched controls”) and 11 nonmatched HIV-negative blood donors. Here, CD4 is linked to lymphoma.