Immunosuppression induced by the human immunodeficiency virus (HIV) increases the risk of developing non-Hodgkin's lymphoma (NHL). As the hepatitis C virus (HCV) has been implicated in the development of B cell lymphomas, we compared the incidence of systemic NHL during HIV infection compared to HIV and HCV co-infection. Of 5,832 individuals studied during the era of highly active anti-retroviral therapy (HAART), 102 patients were diagnosed with systemic NHL. The incidence of systemic NHL was 6.9 of 10 4 patient years during HIV infection compared to 7.1 of 10 4 patient years during HIV alone (p ¼ 0.9). In this immunocompromised patient population, there was no association between HCV infection and an increased risk of lymphoma. ' 2005 Wiley-Liss, Inc.Key words: hepatitis C; HIV; non-Hodgkin's lymphomaThe use of HAART has resulted in marked declines in HIVrelated morbidity and mortality, by mainly reducing the incidence of opportunistic infections 1 High grade B-cell non-Hodgkin's lymphoma (NHL) is an AIDS defining illness, having first been described in association with HIV in the early 1980s. 2 NHL occurs at a greatly increased frequency in HIV-infected populations and despite a reduction in its incidence during the HAART era, it remains the second commonest AIDS defining malignancy after Kaposi's sarcoma. [3][4][5][6][7] Many of the 200 million individuals infected with HCV and the 50 million individuals infected with HIV are co-infected with both viruses. [8][9][10] As well as shared routes of transmission, there are many interactions between these small RNA viruses in which infection is often characterised by parenteral transmission followed by a seroconversion illness and then a long latent period lasting many years. 11 Once established, HIV-1 infection seems to increase the persistence of the hepatitis C virus, the level of HCV RNA and progression of HCV-related disease. [12][13][14][15][16] There are conflicting reports concerning whether HCV infection alters the course of HIV. 17 Some studies before the advent of highly active anti-retroviral therapy (HAART) suggested that survival and progression to AIDS were the same in patients with HCV/HIV co-infection as in those with HIV alone 18,19 whereas others suggested an adverse effect. 13,20 In the pre-HAART era, HCV infection was deemed an irrelevance as it was HIV that affected morbidity and mortality. With the increased life expectancy of individuals with HIV, however, HCV has emerged as a problem that adversely affects survival of individuals with HIV. 8,16,21 HCV is a well-established risk factor in the etiology of hepatocellular carcinomas and of mixed cryoglobulinemia Type II, a condition that can evolve to malignant lymphoma in 8-10% of affected cases. 22 In the non-HIV setting, data from case-case comparisons and case-control studies indicate several-fold higher prevalence of HCV infection (as indicated by the presence of antibodies or HCV RNA) among B cell lymphoma patients compared to control populations. 23 Attempts have been made to evaluate an...