The pollinosis is the Type I allergic reaction, in which IgE antibodies evoked by exposure to antigenic pollen. Following a reaction between the pollen antigen and the IgE antibody, which attaches to the cell membrane of tissue mast cells or blood basophils, certain vasoactive amines are released from those cells, producing symptoms that may be generalized or localized. The diagnosis of pollinosis is based on a detailed history with seasonal regularity and skinprick test, RAST(Radioallergosorbent test) or CAP system(Fluoroimmunoassay), and nasal provocation test can be used as confirmation. For accurate diagnosis and adequate treatment of pollinosis, the ecological and botanical considerations of the allergenic plants and pollens in the area where the patients live are very important. From Jan. 1st in 1993 to Dec. 31th in 1994, authors had observed the causative pollens of 159 patients with the main pollinosis managed at the allergy clinic of Maryknoll General Hospital, and had surveyed the characteristics and distributions of the main plants and pollens causing the pollinosis. The results were as follows : 1) The main causative pollens were tree pollens and weed pollens. Taxonomically,most tree pollens belonged to the order, Fagales and most weed pollens belonged to the family, Compositae. 2) The most common pollen causing the pollinosis was Alnus species(Alder). 3) The main offending tree-pollens were Alnus species(Alder), Betula species(Birch), Corylus species(Hazel), Quercus species(Oak), and Fagus species(Beech). 4) The main offending weed-pollens were Artemisia species(Sagebrush or Mugwort), Chrysanthemum species(Chrysanthemum), and Ambrosia species(Ragweed). 5) The microscopic appearances of pollens belonged to the same genera were very similar to each other.