The importance of sexual transmission in the epidemiology of hepatitis G virus (HGV) and hepatitis C virus (HCV) was evaluated in two groups of HIV-1-positive Lebanese patients. Members of one group (90 patients) were HIV-1-infected via sexual route and denied intravenous drug (IVD) use, while members of the other group (28 patients) became HIV-1-infected parenterally and confessed frequent IVD use. The overall prevalence of HGV infection was relatively high in both groups and with no statistically significant difference between them (28% among IVD users vs 32% among the non-IVD users) despite the fact that non-IVD users were significantly older (32.7±1.7 years) than the IVD users (24.0±1.4 years) (P < 0.01). Conversely, there was a clear association between IVD use and HCV infection (25% for IVD users vs 7% for non-IVD users) despite the significantly lower age of the IVD users. These results point to the efficient transmission of HGV via the sexual route, while the transmission of HCV is mainly via the parenteral route. CD4+ lymphocyte counts were known on only 82 HIV-1-infected patients. Although the number of HGV-RNA-positive patients (three) was small compared with anti-HGV-positive patients (24), a relationship was not found between CD4+ lymphocyte counts and the presence of HGV-RNA in the HIV-1-positive patients. The role of HGV in causing significant liver disease is still under dispute.