The diagnosis of the acute exanthems and other acute infectious and contagious diseases of childhood may at times be simple and obvious. In some cases, however, one's diagnostic ability is taxed to the utmost, and any diagnostic aid, such as a characteristic blood picture, is welcome. Thus the leukocytosis and relative lymphocytosis present in pertussis are frequently important factors in establishing the presence of that disease. The leukopenia of rubeola is also well known. The eosinophilia of scarlet fever, however, seems to be a greatly neglected phenomenon. Not only is there widespread ignorance of the presence of eosinophilia in scarlet fever, but its application as a diagnostic aid seems to be practically nonexistent. Doubtless this is due in part to the relative paucity of literature on the subject. Pediatric textbooks in general devote a scant line or two to the subject and vary considerably in the information offered.Only a