A new lignan glycoside, (7'S, 8'R)-dihydrodehydrodiconiferyl alcohol-4'-O-beta-D-xylopyranoside (1), named cornuskoside A, was isolated from the fruits of Cornus kousa. The structure of the compound was determined by spectroscopy, including FABMS, UV, IR, (1)H-and 13C-NMR, DEPT and 2D-NMR (COSY, HSQC, and HMBC). This new lignan glycoside, along with its aglycone, (7'S, 8'R)-dihydrodehydrodiconiferyl alcohol (3), and another lignan with a similar skeleton, (-)-balanophonin (2), which have been previously isolated from this plant, were evaluated for cytotoxicity in human cancer cell lines such as HeLa, MCF-7, SK-MEL-5, and SK-OV-3. Compounds 2 and 3 showed cytotoxicity against HeLa [IC50 = 29.1 microM (2), 45.5 microM (3)], MCF-7 [IC50 = 29.2 microM (2), 28.2 microM (3)], SK-MEL-5 [IC50 = 31.3 microM (2), 32.3 microM (3)], and SK-OV-3 [IC50 = 33.4 microM (2), 43.4 microM (3)] cell lines.