The study aims to monitor the alder pollen season in selected Polish cities: Bialystok, Cracow, Lublin, Olsztyn, Opole, Piotrkow Trybunalski, Sosnowiec, Szczecin, Warsaw, Wroclaw and Zielona Gora in 2021. Pollen concentrations were recorded by volumetric method using a Burkard-type sampler operating in a continuous volumetric mode. Alder pollen season, defined as the period with 98% of the annual total catch, started in 3rd decade of February in all monitoring sites. There was a marked variation in duration of the season between the sites. It lasted from 31 in Cracow to 54 days in Bialystok (38 days on average). The highest peak daily alder pollen concentrations were observed in Wroclaw (1879 grains/m3) on February 26th). The longest exposure to high concentrations of alder pollen, lasting 22–24 days, was detected in Zielona Gora, Piotrkow Trybunalski and Olsztyn. The alder pollen season in 2021, compared to the previous year, was longer, with higher average sum of daily concentrations over the season, higher maximum daily concentrations and longer exposure to high pollen concentrations at most monitoring sites.