In Switzerland, tropical pulmonary eosinophilia occurs in subjects who have stayed in endemic areas of lymphatic filariasis (mainly India and South East Asia) and in adopted children from these countries. Clinically, the disease manifests itself as asthmatic bronchitis or pneumonia. Blood eosinophilia is regularly present and chest X-ray shows various types of infiltrate. Tropical pulmonary eosinophilia is distinguished from Loeffler's syndrome by (a) the severe and protracted course, (b) measurable antibodies against filarial antigens, and (c) the therapeutic response to diethylcarbamazine. The pathogenesis of both syndromes may be explicable by the fact that soluble parasitic allergens bind to cellules of the respiratory tract and induce hypersensitivity reactions under the influence of reagins. Both syndromes must be differentiated from parasitoses of the lung tissue, from side effects of drugs, and from allergic, non-parasitic bronchial asthma. The clinical aspects of both syndromes are illustrated by three cases.