Pollinosis is now called seasonal allergic rhinitis by the international terminology but pollinosis includes many other symptoms and so we will use the term Ambrosia pollinosis in this article. The characteristics of ragweed pollinosis are: severity, duration from August to September and the presence of asthma and/or tracheitis in about 50% of cases. Ambrosia: phanerogam, dicotyledon, annual plant, monoic. In France, fields in the mid Rhône Valley are covered with Ambrosia artemisiifolia L. but Ambrosia trifida L. is rare. The French Foundation For Ragweed Study (AFEDA) reports here 30years of clinical and epidemiological studies, involving considerable expense, that describe the geographic distribution of this pollinosis in Europe, and applies a predictive model of Ambrosia pollination to an appropriate treatment thanks to a sensitive sensor (Cour pollen trap). The spreading of Ambrosia is partly due to the regulations of the Common Agricultural Policy. There are numerous allergens; recently the major allergen of mugwort has been identified in ragweed. Profilins cause hypersensitivity reactions to certain foods. Genetic predisposition to developing this pollinosis is discussed because sometimes: the disease starts late in life, no personal or family history of atopy is found, immunoglobulin levels are low. Some publications have discussed a genetic predisposition to allergies to Juniperus ashei (United-States) and Cryptomeria japonica (Japan). The clinical efficacy of sublingual specific immunotherapy is well established and well accepted by patients.