Human herpesvirus 8 (HHV-8) infection is common in Africa. We examined the distribution of HHV-8 within families in rural Tanzania to determine routes of spread. HHV-8 infection was assessed by measuring antibody reactivity with a K8.1 (lytic-phase antigen) immunoassay. The prevalence increased from 3.7% (1/27) among infants to 58.1% (36/62) among children aged 3-4 years and 89.0% (65/73) among adults aged > or =45 years. Women with HHV-8-seropositive husbands had a 7-fold risk for infection (odds ratio [OR], 6.9; 95% confidence interval [CI], 1.9-25.3). HHV-8 seropositivity in children was associated with having at least 1 seropositive first-degree relative (OR, 14.7; 95% CI, 5.9-43.1), a seropositive mother (OR, 7.4; 95% CI, 3.2-16.8), a seropositive father (OR, 4.8; 95% CI, 2.3-10.1), or a seropositive next-older sibling (OR, 4.2; 95% CI, 1.9-9.4). Our data are consistent with the occurrence of HHV-8 transmission within families, from mothers and other relatives to children via nonsexual routes and between spouses via sexual routes.