Flavonoid chemistry and chromosome numbers ofFallopiasect.Pleuropteruswere examined to gain insights into the relationships among the taxa within the section. Twenty-seven flavonoid compounds were isolated and identified from taxa of sectionPleuropterus; most of them were glycosylated derivatives of the flavonols kaempferol, quercetin, and myricetin, and of the flavones apigenin and luteolin. The flavonoid data appear to be useful for taxon delimitation, because all taxa examined are readily distinguished by their flavonoid profiles. In addition, the flavonoid data revealed the presence of two chemical entities in the section; the first group includesFallopia multiflora(Thunb. ex Murray) Haraldson (with varieties) andFallopia ciliinervis(Nakai) Hammer and the second group includesFallopia baldschuanica(Regel) Holub andFallopia koreanaOh & Kim. These groupings suggested by the flavonoid chemistry are well-corroborated by the chromosome number of these taxa. Taxa of the first group have chromosome numbers based onx= 11 (2n= 22, 44), whereas chromosome numbers of the second group are based onx= 10 (2n= 20). Our counts forF. multifloravar.hypoleuca(Nakai ex Ohwi) Yonekura & H. Ohashi (2n= 44),Polygonum multiflorumThunb. ex Murray (=F. multiflora) var.angulatumS.Y. Liu (2n= 22),F. ciliinervis(2n= 22), andF. koreana(2n= 20) are the first reports for these taxa. Based on these results, and in conjunction with morphological evidence, it is strongly suggested that the two groups of sectionPleuropterusrevealed by this study are not closely allied and may represent separate lineages in the genusFallopia.Key words: Polygonaceae,Fallopiasect.Pleuropterus, flavonoids, chromosome number, chemotaxonomy.