The MeOH extracts of the ground part and the root of Boenninghausenia japonica NAKAI showed inhibitory activity against tumor cell growth. Fractionation of the extracts has resulted in isolation of 1,3-dihydroxy-4-(2′-hydroxy-3′-hydroxymethyl-3′,4′-epoxy-butyl)-N-methylacridone, 1,3-dihydroxy-4-[(Z)-3′-hydroxy-3′-methyl-buten-1′-yl]-N-methylacridone, 3-(1′,1′-dimethylallyl)-7-hydroxy-8-methoxy-2H-1-benzopyran-2-one, casegravol, cis-casegravol, and edgeworin in addition to 9 compounds reported from B. japonica and B. albiflora. The isolates from this plant and some related compounds were tested for antiproliferative activity against human gastric adenocarcinoma (MK-1), human uterus carcinoma (HeLa), and murine melanoma (B16F10) cells.