The classification of the predominantly Neotropical Eupatorieae depends upon the circumscription of the core genus Eupatorium. The recently proposed narrowing of Eupatorium to ∼42 species in eastern temperate North America, Europe, and eastern Asia was tested with phylogenetic analysis of nucleotide sequence variation in the internal transcribed spacer (ITS) region of nuclear ribosomal DNA. A total of 40 samples (36 species) of Eupatorieae was analyzed. Several species from North America, South America, and Eurasia that were formerly recognized within a large Eupatorium s.l. (sensu lato) were included in the study. Other taxa included were representative of the majority of the subtribes native to eastern temperate North America. Parsimony analysis supported the contention that Eupatorium be defined narrowly and suggested that Eupatoriadelphus is distinct. The tree topology suggested that Eupatorium and Eupatoriadelphus share a common North American ancestor with Liatris relative to other Eupatorieae. It was apparent that the presumed sister taxa in Eupatoriinae from South America belong to a different clade. These results suggest that, following initial divergence in North America, Eupatorium reached Europe via dispersal during the late Pliocene with subsequent radiation in Asia.