The recently discovered GBV-C/hepatitis G virus (GBV-C/HGV) has been shown to be parenterally transmitted. The occurrence of community-acquired GBV-C/HGV infections has also been reported. In order to study the variations with age of the GBV-C/HGV prevalence, sera from 268 individuals without liver disease, aged 0-80 years, and living in the city of Rio de Janeiro, Brazil, were tested by reverse transcription-nested polymerase chain reaction for the presence of GBV-C/HGV RNA. Age-specific seroprevalence was low (2.3%) among children under the age of 10, reached a maximum of 18% in young adults (21-30 years), and declined in older age groups. Among 170 blood donors aged 18-60, the rate of individuals with antibodies against the viral envelope E2 protein increased with age, from about 6% between the ages of 18 and 24 to about 35% for individuals from the age of 43 to 60. The results suggest that sexual transmission of GBV-C/HGV might occur and that the virus could be eliminated after a long period of infection. The nucleotide sequences of GBV-C/HGV genome fragments, 422 base pairs (bp) in the E2 region and 354 bp in the nonstructural 5 region, were determined for 11 and 31 isolates, respectively. Phylogenetic tree based on concatenated E2 + NS5 sequences showed that Brazilian strains belonged to three clusters, two of which were previously characterized as genotypes 1 and 2.