Aqueous extracts of Ilex paraguarariensis (mate-chimarrão), a species that belongs to the Aquifoliaceae family, were analyzed for the presence of genotoxic, mutagenic, and clastogenic activities through bacterial trials based on the induction of the SOS functions, as well as in human lymphocytes in vitro and in mammalian cells in vivo. The extracts of mate-chimarrão were genotoxic, as assessed by lysogenic induction in Escherichia coli, and they also induced mutagenesis in Salmonella typhimurium. They addition of S9 microsomal fraction, catalase, thiourea, or dipyridyl counteracted the genotoxic activity of mate-chimarrão, suggesting that oxygen reactive species play an essential role in the genotoxicity of mate-chimarrão extracts. The extracts were not clastogenic in vivo (bone marrow cells of rats) in our experimental conditions, but we have observed an increased frequency of chromosomal aberrations in mate-chimarrão-treated human peripheral lymphocytes. Our results suggest that a high consumption of mate-chimarrão can potentiate carcinogenesis in the human oropharynx and esophagus.