Hyposmia is a common symptom in allergic rhinitis. However, little is known about differences in the olfactory function of patients with seasonal or perennial allergy. A prospective controlled study was performed on 28 patients with allergic rhinitis to grass pollen and on 47 patients with allergic rhinitis to mites. Sixty-six healthy volunteers served as a control. Olfactory function was evaluated by a modified Connecticut Chemosensory Clinical Research Center testing procedure for threshold, identification, and discrimination. The grass pollen-allergic patients were tested prcseasonally and after 3 weeks of intraseasonal grass pollen exposure: the mite-allergic patients and the volunteers were tested once. In the mite allergies, olfactory threshold, identification, and discrimination tests were significantly worse than in the volunteers (all P<0.0001). In the grass pollen allergies, the results in olfactory identification and discrimination tests were not different from the controls if tested out of the season (both F>0.05). However, in threshold testing (P=0.0139), the results were worse. Intraseasonally, the grass pollen allergies showed a significant decrease in threshold, identification (both P<0.001), and discrimination testing (P=0.0()29). If the intraseasonal pollen allergies were compared to the mite allergies, they showed better results in identification (F=0.(X)87) and threshold (F<0.001) tests, but worse results m discrimination testing (P=0.(XM)2). Therefore, the different kind of allergen exposure seems to result in a different pattern of allergic olfactory dysfunction.