Pollen allergy is the most typical form of allergic diseases. The increase in its frequency during recent years is most evident. The disease was rare when it was first described 150 years ago as summer hay fever. In 1876 Blackley wrote that the frequency of pollen allergy was increasing, especially in industrialized countries. Nowadays the prevalence of pollen allergy is well above 10%. Among the patients predominate the inhabitants of large cities, but not rural residents. Allergy develops as a result of multiple interactions between the human immune system and the environment. The current state of knowledge does not permit precision about the importance of genetic and environmental factors. It appears that although genetic control is very important, the environmental influences seem most important in determining the specific allergens to which a person will develop hypersensitivity. The adjuvant factors of the environment play an important role in the increasing problem of pollen allergy in large cities. Air pollution such as sulphur dioxide, ozone, nitrogen oxides, occupational dust, organic and inorganic suspended particulate matter, tobacco smoke, etc., can cause an increase in airway reactivity through mechanisms dependent on or independent of specific IgE antibodies. By irritating the airways and skin, pollen grain penetration into the human body can increase. Defective permeability of tissue surface can increase the risk of infection. Air pollution may also increase the development of pollen allergy through a direct influence on the pollen grains (on their enzymes, plant growth etc.). Lastly, the pollen grains may carry on their surfaces the various substances which are known as environmental allergens.(ABSTRACT TRUNCATED AT 250 WORDS)