The genus includes all the purple-flowering North American species of the genus Eupatorium as traditionally defined. Eupatorium has recently undergone some revision and has been broken up into smaller genera. Eutrochium is the senior synonym of Eupatoriadelphus. Eupatorium in the revised sense (about 42 species of white-flowered plants from the temperate Northern hemisphere) is apparently a close relative of Eutrochium. Another difference between Eutrochium and Eupatorium is that the former has mostly whorled leaves and the latter mostly opposite ones. Eupatorium and Eutrochium are both placed in the subtribe Eupatoriinae, but South American plants which have sometimes been placed in that subtribe, such as Stomatanthes, seem to belong elsewhere in the tribe Eupatorieae.