To investigate the prevalence of hepatitis G virus (HGV/GBV-C) in patients with liver disease and to confirm its hypothesized ability to cause liver damage, we studied 130 subjects; 61 had chronic hepatitis C virus infection and 69 had acute hepatitis of either defined etiology (n = 57) or of unknown origin (n = 12). Positivity for HGV/GBV-C RNA was detected in 10 of the 61 subjects with chronic hepatitis C (16.3%) and in 11 of the 57 subjects with acute hepatitis of defined etiology (19%), whereas we failed to detect HGV/GBV-C viremia in subjects with hepatitis of non-established etiology. Patients exhibiting positivity for HGV/GBV-C RNA were found to be comparable to those exhibiting negativity for HGV/GBV-C RNA in terms of both liver function tests and Knodell's score (in liver biopsies); the affect of HGV/GBV-C infection on the biohumoral and histological activity in patients with chronic hepatitis C therefore appears to be minimal or absent. Similar clinical features were observed in patients with acute hepatitis of known etiology whether they were positive or negative for HGV/GBV-C RNA. However, long-term clinical studies are still required to clarify the actual impact of HGV/GBV-C co-infection. In our geographic, i.e., a region or north-east Italy, HGV/GBV-C infection appears to be strictly related to intravenous drug use, and this agent does not seem to be responsible for acute hepatitis of unknown etiology; other etiological agents are probably involved.