In early 2017, Southeast Brazil faced a sylvatic yellow fever (sYF) outbreak. We received carcasses of free-living NHP collected in a neighborhood (Vila Toninho) of São José do Rio Preto (São Paulo state), where our research group conducts a prospective dengue cohort study in the population of the neighborhood. All the animals were found dead and the first suspicion was that YFV might have been the cause of death. Some of the animals had signals of trauma caused physical aggression.