A methanolic extract of the fruits of AMOMUM TSAO-KO (Zingiberaceae) significantly attenuated nitric oxide production in lipopolysaccharide-simulated BV2 microglia. Two new bicyclic nonanes characterized as 6,7-dihydroxy-indan-4-carbaldehyde ( 1) and 6-hydroxy-indan-4-carbaldehyde ( 2) were isolated with the eleven known compounds 6,7-dihydroxy-3,7-dimethyloct-2-enoic acid ( 3), tsaokoin ( 4), isotsaokoin ( 5), 8-oxogeraniol ( 6), P-menth-1-ene-5,6-diol ( 7), 3alpha-hydroxycarvotagenone ( 8), tsaokoarylone ( 9), 1,7-bis(4-hydroxy-3-methoxyphenyl)-4,6-heptadien-3one ( 10), (+)-hannokinol ( 11), MESO-hannokinol ( 12) and hannokinin ( 13), from the fruits of A. TSAO-KO using bioactivity-guided fractionation. All thirteen compounds significantly inhibited lipopolysaccharide-induced nitric oxide production in BV2 microglial cells at concentrations ranging from 1 microM to 100 microM.