Our knowledge of eosinophil leucocytosis is still of comparatively recent date. After Ehrlich demonstrated the constant increase of the eosinophil cells in leukaemia a considerable time elapsed before an eosinophilia was found in other diseases, an eosinophilia however that differs in its essential traits from the leukaemic type. To Friedrich Mueller we owe the first researches in this direction, at whose suggestion Gollasch investigated the blood of persons suffering from asthma; in which he was able to demonstrate a considerable increase of the eosinophil cells. This was followed by the researches of H. F. Mueller and Rieder, who discovered the frequency of eosinophilia in children, and its presence in chronic splenic tumours; further by the well-known work of Ed. Neusser, who observed a quite astounding increase of the oxyphil elements in pemphigus, and by the almost simultaneous analogous observations of Canon in chronic skin diseases. From amongst the flood of further papers upon this condition we will only mention the comprehensive account of the subject by Zappert. By =eosinophilia= we understand an =increase only of the polynuclear eosinophil cells in the blood=. Confusion of this form of leucocytosis with leukaemia is quite impossible, because a good number of characteristic signs are necessary for the diagnosis of the latter, as we shall have to explain in the next section. The presence of mononuclear eosinophil cells in the blood should not be regarded, as is the case in many quarters, as an absolute proof of leukaemia, for they are also found in isolated cases of ordinary leucocytosis. The increase of eosinophil cells is not always relative, but may be absolute. The relative number, normally 2 to 4% of all leucocytes, rises in eosinophilia to 10, 20, 30% and over; in a case described by Grawitz 90% indeed was found. The thorough researches of Zappert, carried out on moist preparations by a suitable method, are particularly instructive with regard to their absolute number. As the lowest normal value he gives 50-100 eosinophil cells per mm.^{3}, as mean value 100 to 200, as a high normal value 200-250. The highest absolute number he has ever found was 29,000 per mm.^{3} in leukaemia, the highest number in simple eosinophil leucocytosis 4800 (in a case of pemphigus). Reinbach indeed once found about 60,000 eosinophil cells per mm.^{3} in a case of lymphosarcoma of the neck with metastases in the bone-marrow. Polynuclear eosinophil leucocytosis, apart from the form observed in healthy children, occurs in varied conditions, and for comprehensiveness we divide them into several groups. We distinguish eosinophilia: 1. =In bronchial asthma.= Increase of the eosinophil cells of the blood, often considerable, amounting to 10 and 20% and more has been regularly found, first by Gollasch, later by many other observers. (For the special clinical course of the eosinophilia in asthma see below.)