ETHNOPHARMACOLOGICAL RELEVANCE: Plants are widely as antidiabetics. The study of these plants is essential because many of them may have undesirable effects, such as acute or chronic toxicity; or their use may even delay or discourage the adoption of the proper and effective treatment. MATERIALS AND METHODS: The present study surveyed the plant species that are popularly used to treat diabetes mellitus in the state of Rio Grande do Sul in southern Brazil. Sixteen ethnobotanical surveys performed in the state were consulted, and the species used to treat diabetes were listed. For species cited in at least two of the studies, scientific data related to antidiabetic activity were searched in the ISI Knowledge database. The scientific binomial of each species was used as keywords, and data found in review papers were also included. RESULTS: A total of 81 species in 42 families were mentioned; the most important families were Asteraceae and Myrtaceae. Twenty eight species were cited at least twice as being used to treat diabetes in the state. For 11 of these, no scientific data regarding antidiabetic activity could be located. The species most frequently mentioned for use with diabetes were Syzygium cumini (Myrtaceae) and Bauhinia forficata (Fabaceae), in 12 studies each, followed by Sphagneticola trilobata (Asteraceae), in six studies; and Baccharis trimera (Asteraceae), Bidens pilosa (Asteraceae), Cynara scolymus (Asteraceae), and Leandra australis (Melastomataceae) in four studies each. Bauhinia forficata and Syzygium cumini have been studied in more detail for antidiabetic activity. CONCLUSIONS: A considerable number of plant species are traditionally used for the treatment of diabetes melitus in the Rio Grande do Sul State. The majority of those plants that have been studied for antidiabetic activity showed promising results, mainly for Bauhinia forficata and Syzygium cumini. However, for most of the plants mentioned, the studies are not sufficient to guarantee the efficacy and safety in the use of these plants in the treatment against diabetes.