The possibility that impaired production of bronchoprotective factors contributes to the pathogenesis of asthma cannot be excluded. Prostaglandin E2 (PGE2) could be such a factor. It is a dominant cyclo-oxygenase product of airway epithelium and smooth muscle; it has inhibitory effects on inflammatory cells and pathways involved in bronchoconstriction at concentrations known to occur in the airway; inhalation of PGE2 has considerable bronchoprotective effects in patients with asthma; and manoeuvres that increase or decrease endogenous production of PGE2 have beneficial and deleterious effects on airway function.