The authors performed allergen research on patients reported hay fever symptoms during the poplar pollen season (March), and in patients with hay fever symptoms during the time of the year that the seed hairs of the poplar trees are blowing in the air (May). Skin prick tests (Epipharm) and serum specific IgE tests (Epignost IgE Quick and Phadezym Populus deltoides RAST, Pharmacia) were performed on the basis of the pollen calendar of Szeged region on 30 patients. The pollen containt of the air was measured by means of a Lanzoni sampler. According to the pollen calendar of our region a large amount of grass pollens could be found in the air at the same time as the seed hairs of poplar trees are present (in May). The season of poplar pollen is in March in this area. Poplar pollen sensitivity was found on 8 patients. This is 6.8% of the total number of hay fever patients. They were found to be sensitive to other tree pollens too. The 23 patients complaining about hay fever symptoms in May, during the flaying of the cottons of the poplar trees in the air were all found to be sensitive to grass pollens. On the basis of our results the poplar pollen sensitivity is a relatively rare cause of hay fever. Our patients having complained about the seed hairs were all found to be sensitive to grass pollens. It seems that the grass pollens are the real cause of their disease.