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The tree flora of seasonally dry tropical forests (SDTF) of eastern tropical and subtropical South America was investigated according to two main aspects: (a) the variations in floristic composition were analysed in terms of geographical and climatic variables by performing multivariate analyses on 532 existing floristic checklists; and (b) the links among different seasonally dry forest formations, Amazonian forests and cerrados (woody savannas) were assessed. Analyses were performed at the species, genus and family levels. There was a strong spatial pattern in tree species distribution that only receded and allowed clearer climate-related patterns to arise when either the geographical range was restricted or data were treated at the genus and family levels. Consistent floristic differences occurred between rain and seasonal forests, although these were obscured by strong regional similarities which made the two forest types from the same region closer to each other floristically than they were to their equivalents in different regions. Atlantic rain and seasonal forests were floristically closer to each other than to Amazonian rain forests but north-east rain and seasonal forests were both closer to Amazonian rain forests than each other, though only at the generic and familial levels. Atlantic seasonal forests also share a variable proportion of species with caatingas, cerrados and the chaco, and may represent a transition to these open formations. Increasing periods of water shortage, with increases in soil fertility and temperature are characteristic of a transition from semideciduous to deciduous forests and then to the semi-arid formations, either caatingas (tropical) or chaco forests (subtropical), while increasing fire frequency and decreasing soil fertility lead from seasonal forests to either cerrados (tropical) or southern campos (subtropical). The SDTF vegetation of eastern South America may be classified into three floristic nuclei: caatinga, chaco and Atlantic forest (sensu latissimo). Only the last, however, should be linked consistently to the residual Pleistocenic dry seasonal flora (RPDS). Caatinga and chaco represent the extremes of floristic dissimilarity among the three nuclei, also corresponding to the warm-dry and warm-cool climatic extremes, respectively. In contrast to the caafinga and chaco nuclei, the Atlantic SDTF nucleus is poor in endemic species and is actually a floristic bridge connecting the two drier nuclei to rain forests. Additionally, there are few grounds to recognize the Atlantic nucleus flora as a clearly distinct species assemblage, since there is a striking variation in species composition found throughout its wide geographical range. Nevertheless, there is a group of wide-range species that are found in most regions of the Atlantic nucleus, some of which are also part of the species blend of the caatinga and chaco floras, though the latter plays a much smaller part. We propose that it is precisely this small fraction of the Atlantic nucleus flora that should be identified with the RPDS vegetation.