ETHNOPHARMACOLOGICAL RELEVANCE: This article presents the results of a study on wild plant pharmacopeia and medical ethnobotany of the Wichí people of the South American Gran Chaco region, where native forest still persist. Few previous works on the former subjects exist, with only partial information. Traditionally, shamans are in charge of healing serious diseases and wild plants are used for treating minor problems. Some ethnobotanists believe that much of the present pharmacopeia of native peoples of the Gran Chaco comes from the Criollos (local non-native people) and that medicinal plants and uses would have been much fewer in the past. AIMS OF THE STUDY: The study aims to: a) document the wild medicinal plants used by the Wichís and the associated ethnobotanical knowledge, b) discuss the use of medicinal plants in the current sanitary, epidemiological and ethnomedical context of the Wichís, and c) analyse the similarities between the medicinal plants and uses of the Wichís and those reported for the Criollos of the study area, for the We'enhayek (Wichís of Bolivia) and for other indigenous peoples of the Gran Chaco. All of them will help to answer a general research question: are medicinal wild plants of the Wichís present pharmacopoeia long-dated or are they relatively new adquisitions? MATERIALS AND METHODS: Original data were obtained between 2005 and 2017 from 51 informants through open and semi-structured interviews, participant observation, "walks-through-the-forest" technique and gathering of plant vouchers. Consensus of Use (CU) per species, use, ailment and use-category were calculated. Species and uses of the Wichís and those reported for other groups of the region were compared confronting the data, a similarity index (Sorensen) was calculated and a cluster analysis was carried out. An interpretative analysis of the results was performed. RESULTS: Original data consist of 408 applications for which 115 plants of 48 botanical families are used for treating 68 ailments or symptoms. Species are mainly used for treating prevalent health disorders in Wichí epidemiological context (skin and digestive disorders, fever, respiratory affections) and feminine issues. CONCLUSIONS: The similarity of the current Wichí pharmacopoeia with the other ethnic groups of the region seem to respond more to a geographical proximity than to cultural affinity. Much (but not all) of their pharmacopoeia seem to be novel, resulting from a permanent seek for solutions to both old and new health problems in the native forests. Many medicinal plants, but not specific uses, seem to come from the neighbouring Criollos, whilst traditional remedies seem to be simultaneously preserved. Hence, I propose that the ethnobotanical "diversification hypothesis", slightly modified, applies in the case of wild medicinal plants of the Wichís, as new wild medicines are being added to the old ones to fill therapeutic vacancies that for several sociocultural and historical reasons appear in their traditional pharmacopoeia. Altogether, results provide novel information of interest for ethnopharmacology, medical ethnobotany and related disciplines, expand the knowledge of Wichí pharmacopoeia and constitute a baseline for future diachronic and cross-cultural studies in the Gran Chaco region.