The causative relationship of pollens to hay-fever has become so well established that it appears almost superfluous to offer additional evidence. The experience of the past year in the correspondence of the American Hay-Fever-Prevention Association, however, has shown that there are still many who doubt that hay-fever is caused by pollen, and that there are many more who believe that pollen is only one of the exciting causes of this disease. Under the circumstances, a brief review of the most important proofs of this relationship will be of advantage. In hay-fever, as in many other pathologic conditions, we have to consider, in addition to the exciting cause, the predisposing factors. In the greater portion of the United States there are at certain seasons many pollens in the atmosphere, which are inhaled by all in the course of normal respiration. Of this number, only a certain proportion of