Antiinflammatory activities of heat-treated Cassia alata leaf extract and kaempferol 3-O-gentiobioside (K3G) isolated from C. alata as an abundant flavonoid glycoside were studied by comparing their activities with the activities of sun-dried C. alata leaf extract. We observed strong inhibitory effects on Concanavalin A-induced histamine release from rat peritoneal exudate cells both in the extracts of heat-treated and sun-dried C. alata leaves. Furthermore, the heat-treated leaf extract exhibited stronger inhibitory effects than the effects of the sun-dried leaf extract at low concentrations in the studies of Concanavalin A-induced histamine release, 5-lipoxygenase inhibition, and also inhibition of cyclooxygenases (COX-1 and COX-2), whereas K3G showed weak inhibitory effects on Concanavalin A-induced histamine release, 5-lipoxygenase, and COX-1. No anti-hyaluronidase effect was detected in any of the materials tested.