Nonspecific hyperreactivity to a number of physical, chemical and pharmacological agents in a common characteristics of allergic subjects, which is strictly related to the inflammatory events consequent to an allergic reaction. Hyperosmolar stimulus has previously been shown to cause an inflammatory response inducing mast cell activation and subsequent mediator release. We investigated the clinical and cytological events following conjunctival challenge using a hyperosmolar agent (i.e. glucose solution) in patients with allergy due to pollen, in patients with allergy due to mites and in healthy individuals in order to investigate the possible existence of 'conjunctival hyperreactivity', similar to the well-known nasal and bronchial nonspecific hyperreactivity in allergic patients. Hyperosmolar stimulus was able to produce an immediate inflammatory response evidenced by clinical symptomatology, inflammatory cell infiltrate and CD54 expression on conjunctival epithelium in all allergic subjects and healthy volunteers. House dust mite-sensitive subjects, but not pollen-sensitive subjects studied out of pollen season, showed conjunctival hyperreactivity as shown by histamine challenge, in that they required lower glucose concentrations to elicit the conjunctival reaction. This is probably related to an underlying although mild, inflammation.