Hypersensitivity of the respiratory system to pollens depends on the dominant kind of pollen grains in the air. It manifests as seasonal allergic rhinoconjunctivitis and asthma. The appearance and the severity linearly correlate with the pollen concentration in the air. In the last few years an increase in the number of people hypersensitive to pollen has been observed in the Osijek-Baranja region. It correlates with the expansion of ragweed and the increase of its air pollen concentrations. Since 2001, pollen and spores are being monitored in Osijek. This project has been initiated by the Association for the Prevention of Allergic Diseases and has been carried out in cooperation with the Regional Office of Public Health. Measurements are done using the Burkard volumetric apparatus according to the predetermined standard and are presented in pollen/m3 of air. Ragweed pollen is found in the air from the beginning of August until the end of September, with the maximum contentrations from the end of August until the middle of September.