In order to validate the use of Crescentia alata (Bignoniaceae) in the traditional medicine of Guatemala as an antiinflammatory remedy, the methanolic (MeOH) extract has been evaluated in vivo for antiinflammatory activity on carrageenin paw edema in rats and in vitro on Escherichia coli lipopolysaccharide- (LPS)-induced nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression in J774.A1 macrophage cell line. This extract exerted in vivo a significant anti-inflammatory activity at the highest dose tested. The same extract showed in vitro an inhibitory activity on inducible nitric oxide synthase expression and on NO formation in LPS-primed J774.A1 cells. Subsequent fractionation and analysis of the extract has led to the isolation and characterization as major constituents of two flavonol glycosides: quercetin 3-O-alpha-L-rhamnopyranosyl-(1->6)-beta-D-glucopyranoside (rutin) 1, kaempferol 3-O-alpha-L-rhamnopyranosyl-(1->6)-beta-D-glucopyranoside (kaempferol 3-O-rutinoside) 2, and flavonol aglycone, kaempferol 3. Their structures were elucidated by spectral methods. The bioassay-directed analysis of flavonols 1-3 indicated that kaempferol (3) was the most active compound contained in the MeOH extract because it reduced in vitro both NO production and iNOS expression in LPS-primed J774.A1 cells, whereas rutin (1) and kaempferol 3-O-rutinoside (2) showed no significant activity. The MeOH extract and all of flavonoids tested did not show in vitro significant cytotoxic effect in J774.A1 macrophage cell line.