Low incidence rates of respiratory diseases are found in agricultural workers although they are exposed to risks peculiar to their employment. These risks are of two types, chemical and mycotic. Chemical risks include the inhalation of oxides of nitrogen on entering the tops of enclosed silos. Mycotic risks arise from the rare group of fungi which actually grow in and invade the lung tissue and from the larger and more common group where there is a specific type of allergy to certain groups of fungi. The cause, incidence and prevention of Farmer's Lung are given.