Human herpesvirus 8 (HHV-8) seroprevalences were determined in two isolated Amazon Amerindian tribes, according to age, gender and familial aggregation. Plasma and serum samples obtained from 982 Amazon Amerindians (664 Tiriyo´ and 318 Waiampi) were tested for antibodies against lytic and latent HHV-8 antigens by using ‘in-house’ immunofluorescence assays. Overall, HHV-8 seroprevalence was 56?8% (57?4% in the Tiriyo´ tribe and 55?7% in the Waiampi tribe). Seroprevalence was independent of gender and increased linearly with age: it was 35?0% among children aged 2–9 years, 51?4% in adolescents (10–19 years), 72?9% in adults and 82?3% in adults aged >50 years. Interestingly, 44?4% of children under 2 years of age were HHV-8-seropositive. No significant differences in seroprevalence between tribes and age groups were detected. It is concluded that HHV-8 is hyperendemic in Brazilian Amazon Amerindians, with vertical and horizontal transmission during childhood, familial transmission and sexual contact in adulthood contributing to this high prevalence in these isolated populations.