Hepatitis G virus (HGV) is a recently described flavivirus present in 1–2% of blood donors. It is transmitted parenterally and is found at high frequency in patients receiving multiple blood products, haemodialysis patients and injecting drug users. Acute infection causes a mild hepatitis in a minority of patients. Many subjects remain persistently infected, but there is little evidence to date that chronic HGV infection causes hepatic or other disease. At present polymerase chain reaction-based techniques are used for diagnosis, although serological tests are being developed. Currently, there is little evidence to suggest that screening of blood donors for HGV infection should be carried out.