Abstract For the prevention of hay fever (H.F.) we must know each year, which are our allergenic plants, and where and when they pollinate. For that purpose we combine: clinical history and skin tests, information collected by a team of skilled botanists (who go out in the field once in two weeks, and determine according to the flower's developmental stage, when its pollination is to be expected) and data (both clinical and aerobiological) collected in previous years. This way we are able to start the administration of preventive medication in time, before the relevant season has started, and till its end. With our preventive treatment schedule, we got the following results in a group of 221 children from the area served by our clinic: in order to be kept H.F. symptoms free, out of the 221: 36, needed only antihistamines, 108, needed in addition cromolyn, and 72, topical steroids, in the remaining 15, medication was not satisfactory, and immunotherapy was applied successfully. We conclude that each H.F. patient should get his own individual preventive treatment schedule, established on the grounds of his personal, and the aerobiological data.