Adult expatriates in countries where hepatitis B virus (HBV) is highly endemic have an increased risk of HBV infection, but little is known about risks to their children or about patterns of spread. The epidemiology of HBV infection was studied among 124 unvaccinated Dutch missionaries and family members who lived in a rural area of Nigeria. Antibodies to hepatitis B core antigen were found in 5 (9.8%) of 51 adults (incidence rate, 1.7 per 1000 person-months at risk [PMAR]) and 9 (12.3%) of 73 children (incidence rate, 2.8 per 1000 PMAR). Vertical transmission of HBV was a likely source of infection in 1 child and was a possible source of infection in 2 others. The prevalence of HBV infection showed strong family clustering (P<.0001), was associated with a history of temporary adoption of Nigerian children (P=.004), and increased with both the number of adoptive children (P=.009) and the total time that these children had stayed in the family (P=.036). Horizontal transmission from adoptive Nigerian children probably played an important role in the spread of HBV infection in this expatriate community.