We investigated the effects of the aqueous extract of Aquilaria agallocha Roxb. (Thymelaeaceae) on the immediate hypersensitivity reactions. The aqueous extract of Aquilaria agallocha stems showed inhibitory effects on passive cutaneous anaphylaxis, anaphylaxis induced by compound 48/80, and histamine release from rat peritoneal mast cells (RPMC). The morphological examination also clearly showed that the extract prevented the degranulation of RPMC in rats. The level of compound 48/80-induced intracellular cAMP in RPMC, when the extract was added, significantly increased about 8-fold at 10 s compared with that of basal cells. These results suggest that the aqueous extract of Aquilaria agallocha stems inhibits the immediate hypersensitivity reaction by inhibition of histamine release from mast cells.