Corylus produces allergenic pollen grains that appear in the air in early spring and cause pollen allergy in sensitive people. The aim of this study was to compare the Corylus pollen seasons in 2021 in the following 11 cities in Poland: Bialystok, Bydgoszcz, Cracow, Sosnowiec, Lublin, Olsztyn, Piotrkow Trybunalski, Szczecin, Warsaw, Wroclaw, and Zielona Gora. This research was conducted using the volumetric method and Burkard or Lanzoni pollen samplers. Pollen season duration was determined by the 95% method. The hazel pollen season in 2021 began relatively late, between February 20 and March 1. The season start was recorded earliest in Zielona Gora, while latest in Olsztyn. The highest values of maximum Corylus pollen concentration were recorded in Sosnowiec (230 P/m3) and Zielona Gora (213 P/m3), whereas the lowest ones in Bialystok (27 P/m3) and Bydgoszcz (54 P/m3). In most of these cities, the maximum daily concentration of Corylus pollen grains was recorded in the third 10 days of February or at the beginning of March and only in Lublin and Bialystok the peak value occurred later, on March 16 and March 26, respectively. The highest risk of allergy in people sensitive to the pollen of this taxon was found in Lublin, Olsztyn, and Zielona Gora. The highest values of the annual pollen integral were determined in Lublin, similarly to the previous years.