During the last five years at the Massachusetts General Hospital, a large group of hypersensitive persons has been studied. The case histories of a number of these persons are so suggestive of the possibility that the hypersensitiveness may have been acquired that it has seemed worth while to review some of them because of the bearing which such a possibility would have on the study of the nature and mechanism of hypersensitiveness in general. Some of these case histories are particularly striking. REPORT OF CASES Case 1. —F. O., a chemist, aged 31, has been employed in the dye laboratory of a woolen mill, where he has worked for three years. For the last year he has had sneezing and asthma whenever he goes into this laboratory, but if he stays away he never has trouble. In his work, he handles a great variety of dyes in powder form. When