The study was carried out to evaluate the effectiveness of the vaccination program against bovine brucellosis, implemented by the State of Rondônia, using prevalence rates as an indicator. The State was divided into three regions. For each region, a pre-determined number of herds were selected and at each herds, blood samples were collected from randomly chosen females with 24 months age or more. Sera from animals were submitted to a serial testing protocol, with screening performed using the buffered acidified antigen test and confirmation by the complement fixation test. A epidemiological questionnaire was administered to each farm property to identify the risk factors associated with the disease. The prevalence rates of infected herds and infected animal at the state were 12.3% [10.3-14.6] and was 1.9% [1.4-2.5], respectively. Between the regions, the prevalence of infected herds varied from 11.6% to 12.8% and the prevalence of infected animals ranged between 1.4% and 2.6%. Bovine brucellosis in Rondônia State is associated with the beef herd, a larger number of cows and the presence of flooded pastures. There was an important reduction in prevalence rates of infected herds and infected animals since 2004, however further reduction could still be achieved with vaccination. Thus, the State should continue its vaccination program, placing emphasis on ensuring the quality of the process. In addition, it should also promote the use of a vaccine that does not induce antibody formation. Additionally, the State must carry out a great effort of education so that producers test breeding animals for brucellosis before introducing them onto their properties and, where possible, prevent calved cows from having access to areas that are flooded. Key words: Bovine brucellosis. Vaccination. Prevalence. Risk factors. Rondônia. Brazil. ResumoO estudo foi realizado para se verificar a eficácia do programa de vacinação contra brucelose bovina implementado pelo estado de Rondônia, utilizando-se a prevalência como indicador. O Estado