A study was made of 30 patients with perennial extrinsic rhinitis sensitized only to dust mites who were treated with cetirizine for 15 days, at a dose of 10 mg/24 hours. The following parameters were evaluated in each patient on day -1 before initiating treatment and on day 15: histamine 1/100 prick-test, Dermatohophagoides pteronyssinus prick-test, physical examination, assessment of nasal edema and rhinorrhea by anterior rhinoscopy, and of the degree of nasal obstruction by active anterior rhinomanometry. Each patient was given a form for daily clinical self-evaluation of the following subjective symptoms: sneezing, rhinorrhea, nasal obstruction, and nasal itchiness. The occurrence of side effects was evaluated. Cetirizine reduced significantly the area of the histamine and D. pteronyssinus papules elicited by prick-test. Clinical symptoms decreased significantly, with sneezing, nasal itchiness and rhinorrhea being greatly relieved, and nasal obstruction, evaluated by means of active anterior rhinomanometry performed before and after 15 days of treatment, being less alleviated. There was scant incidence of side effects in patients treated with cetirizine.