The occurrence of excessive leukocytosis and with or without associated enlargement of peripheral lymph nodes and spleen, and pulmonary symptoms, particularly of asthmatic bronchitis, has been reported for many years, predominantly from the tropics. The term, eosinophilia, however, was first used by Weingarten who reported 81 cases of hypereosinophilia observed since 1934. In his series of cases no etiologic agent was found. In some cases reported earlier by other authors, the available evidence pointed to parasites as the causative factor of the syndrome. De Langen observed several patients with asthmatic attacks, leukocytosis and marked eosinophilia who were infected with Strongyloides stercoralis. Meyers and Kouwenaar found the microfilariae of Filaria malayi in the enlarged lymph nodes of patients with the same hematologic findings. The worms were lying in small abscesses containing eosinophilic leukocytes almost exclusively. In 1945 and 1946 van der Sar and Hartz' reported two instances of tropical eosinophilia in which microfilariae, in all probability Wuchereria bancrofti, were found in enlarged axillary lymph nodes. In these instances the worms were also lying in eosinophilic abscesses or infiltrates. In a third patient, in whom the submental nodes and the nodes in the sulcus bicipitalis dexter were greatly enlarged, no microfilariae had been demonstrated at the time of their report; but they were found subsequently. Meyers and Kouwenaar and van der Sar and Hartz, ' reported that repeated examinations of the blood of their patients failed to reveal the presence of microfilariae. Carter, Wedd and d'Abrera and Soysa and Jayawardena, all working in Ceylon, found larval mites, mostly Tyroglyphus or Tarsonemus, in the sputum of patients having tropical but no enlarged lymph nodes. Their findings were confirmed by van der Sar at Curacao and by Wilson in East Africa. Therefore, it is very probable that the inhalation of dust, containing larval or sometimes adult mites, may also cause the syndrome of tropical eosinophilia. The exact pathogenesis of tropical eosinophilia is not known. Microfilariae may be present in various regions without causing local tissue changes; when such changes are present, they may exist without tissue or marked blood eosinophilia. Furthermore, tropical eosinophilia develops in only a small fraction of the patients harboring microfilariae. It may, however, be safely assumed that hypersensitivity plays an important role. The diagnosis of tropical eosinophilia may be difficult since the microfilariae and larval mites can be found only by