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From the methanolic extract of the flowers of Filipendula vulgaris, a new dimeric flavonol is isolated and identified as quercetinyl 3-O-glucoside-4′-O-4′′′′-quercetin (QI 3-O-glucoside-4′-O-4′′′′-QII) by chemical and spectroscopic methods. Also, six known flavonoids were isolated from the flowers for the first time and identified. The methanolic extract of the flowers (F) and quercetin (7), which were isolated in high concentration, with its synthesized fully methoxylated (8) and ethoxylated (9) derivatives, were investigated against three human carcinoma cell lines; HCT-116, HepG2, and MCF-7. The results showed cytotoxic activities of F and 7 against HCT-116 with IC50 (5.8 and 9.7 μg/mL) relative to that of doxorubicin (IC50 = 21.8 μg/mL), while both 8 and 9 showed activities against HepG2 with IC50 (43.9 and 44.5 μg/mL, respectively) relative to that of doxorubicin (IC50 = 63.3 μg/mL). Furthermore, all of these samples were tested against nontumor fibroblast-derived cell line (BJ-1) and demonstrated no cytotoxicity. [ABSTRACT FROM AUTHOR]