PURPOSE: This study was carried out in Monte Negro (state of Rondônia), a village in the Brazilian western Amazon region, where a University of São Paulo Medical School program for medical student training in rural assistance took place. It aimed to determine the prevalence of hepatitis B virus and hepatitis C virus, to investigate risk factors for infection, and to evaluate the State immunization program against hepatitis B virus in the region. METHODS: The study is a cross-sectional seroprevalence survey, comprising 267 volunteers who answered a comprehensive questionnaire and had blood samples collected, which were analyzed in São Paulo for the presence of antibodies against hepatitis B virus (Hbs Ag, anti-Hbs, and anti-Hbc) and hepatitis C virus using commercial kits. Data were stored in a specific data bank, and the association between seropositivity and potential risk factors was analyzed by means of uni-, bi-, and multi-variate analysis, considering ±5%. RESULTS: The seroprevalence of hepatitis B virus was 61.79% and of hepatitis C virus was 0.38%. Statistical analysis on the data bank showed that the prevalence of hepatitis B virus rose significantly with age, especially after adolescence. Infection was higher in those coming from outside the state of Rondônia. Exposure to vaccination against hepatitis B virus was higher in younger individuals and in those who were born in Rondônia. CONCLUSION: Monte Negro is a highly endemic region for hepatitis B virus but not for hepatitis C virus. Our results also provide indirect evidence indicating a significant improvement in the immunization program in Rondônia in recent years.