It is known that patients on chronic haemodialysis are frequently infected with hepatitis C virus (HCV). It has recently been found that GB virus-C (GBV-C) and hepatitis G virus frequently coinfect patients with HCV. This study aimed at elucidating the clinical implications of GBV-C infection among haemodialysis patients who have and do not have HCV infection. GBV-C RNA was detected in sera of randomly selected 98 anti-HCV-positive and 85 -negative patients on dialysis by reverse transcription-polymerase chain reaction using two sets of amplification primers made from the reported sequences of the non-structural protein 3 and 5' untranslated regions. In these patients, liver function tests were carried out at regular intervals. There were six patients who were coinfected with HCV and GBV-C and three who had only GBV-C RNA. All had a history of past blood transfusion. The onset of mild hepatitis was identified in three HCV-negative patients; elevation of alanine aminotransferase (ALT) following blood transfusion was very mild but recognizable, and aspartate aminotransferase (AST) was higher than ALT. In two of six coinfected patients, the onset of liver disease was recognized with a peak ALT of 72 and 90 IU/L, respectively. Two of these six were Amplicore (HCV-RNA) negative and asymptomatic, two had low-grade HCV viraemia and two moderate-grade HCV viraemia. Of the 98 anti-HCV-positive cases, 41 were thought to have had nosocomial infection of HCV or non-A, non-B virus; none of them had GBV-C. GBV-C RNA was negative in nine patients who had chronic non-A-E hepatitis. GBV-C infection was detected in 6.1% of anti-HCV-positive and in 3.5% of -negative dialysis patients. All had blood transfusion in the past, and there was no evidence of patient-to-patient spread of GBV-C in hospital. The liver disease was very mild and self-limited in GBV-C infection alone. The natural history of coinfected patients may be similar to that of those with chronic HCV infection, but the liver disease appears to be milder.