BACKGROUND Cetirizine is an antihistamine used in the treatment of allergic rhinoconjunctivitis, that has antiallergic activity. OBJECTIVE The aim of this double-blind, placebo-controlled, parallel group study was to evaluate the clinical efficacy and the antiallergic activity of cetirizine, administered either continuously or on demand over a 4-week period of natural allergen exposure. METHODS Twenty patients, with allergic rhinoconjunctivitis due to grass and/or Parietaria pollen, were enrolled. They were randomized into 2 parallel groups: one group received the standard dose of 10 mg cetirizine daily and the other received placebo, all patients were allowed to take an additional daily dose of cetirizine when needed. Variables evaluated were clinical symptoms (recorded on diary cards), number of additional on demand cetirizine doses, nasal inflammatory cells, and pollen counts. RESULTS The results of the present study show that patients treated with continuous administration of cetirizine achieved significant symptomatic relief and inflammatory control (decreases in numbers of infiltrating neutrophils and eosinophils) in comparison to patients treated on demand. CONCLUSIONS Continuous treatment with cetirizine is more effective than on demand treatment. Continuous treatment reduces clinical and inflammatory variables more than symptomatic treatment and the on demand therapy can determine acceptable clinical control, but does not reduce allergic inflammation.