Objective--In order to elucidate the pathogenesis of the radiologic opacity of the sinuses frequently observed in patients with allergic rhinitis, the mechanisms underlying their sinus mucosal swelling were studied clinically.Material and methods--We confirmed the presence of hypoxia in inflamed sinuses and obstruction of the sinus ostium in operated patients with chronic sinusitis by digitally monitoring the oxygen tension. The possibility of radiologic sinus shadow was also investigated after transient obstruction of the natural ostium.Results--The oxygen tension was significantly lower in inflamed than non-inflamed sinuses (p<0.01), irrespective of the presence or absence of allergic rhinitis. In 54.5% of patients without sinusitis, transient obstruction of the middle meatus by gauze packing resulted in the appearance of a pathologic sinus shadow on radiograms obtained after septoplasty and turbinotomy. In both allergic and non-allergic rhinitis, thick opacity was the most frequently encountered pattern (p<0.01).Conclusion--Our study revealed that in the absence of a primary allergic reaction in the sinus mucosa, blocking of the middle meatus and ostium by allergic swelling of the nasal mucosa may induce hypoxia and secondary mucosal swelling in the sinuses.