GB virus-C (GBV-C) and hepatitis G virus (HGV) are recently identified non-A-E hepatitis-associated viruses belonging to the family Flaviviridae. The prevalence of GBV-C/HGV in the general population is high (1.2-3.0%), but data on possible transmission routes are sparse. In this report GBV-C/HGV RNA was detected in a couple by reverse transcription polymerase chain reaction (RT-PCR) using primers deduced from non-structural regions. Neither partner was coinfected with hepatitis B (HBV), hepatitis C (HCV) or human immunodeficiency virus (HIV). The child of the couple tested repeatedly negative for GBV-C/HGV by RT-PCR. The couple had lived in a stable monogamous relationship for 10 years and had never used barrier contraception. Other than sexual risk, factors for transmission were carefully excluded. GBV-C/HGV isolates obtained from the couple were sequenced and phylogenetically analysed together with control GBV-C/HGV isolates. The evolutionary distance between the sequences obtained from the couple (1%) was smaller than between any other GBV-C/HGV sequence. Taken together, the clinical and molecular data provide strong evidence for heterosexual but not vertical transmission of GBV-C/HGV in non-coinfected individuals.