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In Brazil, epizootics among New World monkey species may indicate circulation of yellow fever (YF) virus and provide early warning of risk to humans. Between 1999 and 2001, the southern Brazilian state of Rio Grande do Sul initiated surveillance for epizootics of YF in non-human primates to inform vaccination of human populations. Following a YF outbreak, we analyzed epizootic surveillance data and assessed YF vaccine coverage, timeliness of implementation of vaccination in unvaccinated human populations. From October 2008 through June 2009, circulation of YF virus was confirmed in 67 municipalities in Rio Grande do Sul State; vaccination was recommended in 23 (34%) prior to the outbreak and in 16 (24%) within two weeks of first epizootic report. In 28 (42%) municipalities, vaccination began more than two weeks after first epizootic report. Eleven (52%) of 21 laboratory-confirmed human YF cases occurred in two municipalities with delayed vaccination. By 2010, municipalities with confirmed YF epizootics reported higher vaccine coverage than other municipalities that began vaccination. In unvaccinated human populations timely response to epizootic events is critical to prevent human yellow fever cases. Author Summary: Yellow fever (YF) is a viral hemorrhagic disease that affects humans as well as several species of non-human primates, especially New World monkeys found in South America. Yellow fever virus (YFV) is maintained in a natural cycle involving tree-hole breeding mosquitoes and non-human primates hosts. Because YF is often fatal in susceptible New World monkey populations, sudden die-offs of New World monkeys or epizootics can signal YFV circulation in an environment where humans may be exposed. Surveillance for such events is an important tool to prevent human disease. The state of Rio Grande do Sul in southern Brazil was one of the first states to conduct passive and active surveillance for yellow fever virus circulation in non-human primates. During a rapidly-spreading YF outbreak in 2008–2009, surveillance for epizootics involving non-human primates informed vaccination of human populations in areas previously free of YF. In this study, we analyzed surveillance data to evaluate vaccine coverage, timeliness of notification, investigation and confirmation of epizootic events in relation to occurrence of human cases of YF. Epizootic surveillance is useful for directing the efforts of vaccine distribution. Prevention of YF in susceptible human populations requires immediate risk communication and implementation of vaccine recommendations after the first reports of epizootic activity when YF is suspected. [ABSTRACT FROM AUTHOR]