Azelastine is a phthalazinone derivative with a wide spectrum of pharmacologically relevant activities. Since PAF-acether has been considered to be a potent mediator of asthma, azelastine was assayed for its ability to counteract PAF-acether-induced platelet aggregation, paw edema development and bronchoconstriction. Azelastine exerted a concentration-dependent inhibition of PAF-acether-induced platelet aggregation in human platelet rich plasma with an IC50 of 87 mumol/l and was as effective as ketotifen. PAF-acether-induced paw edema was reduced by intraperitoneal administration of azelastine resulting in an ID50 of 14.4 mg/kg after 2 h. By contrast, ketotifen was not able to inhibit edema development up to a dose of 32 mg/kg i.p. Azelastine and ketotifen, administered intravenously, dose-dependently inhibited PAF-acether-induced bronchoconstriction, starting from the dose of 0.01 mg/kg and resulting in ID50s of 0.03 and 0.02 mg/kg, respectively. These results show that azelastine is endowed with a peculiar anti-PAF-acether action, which may be advantageous in its therapeutic use, in the treatment of asthma.