Twenty-seven pulmonary lesions have been reviewed both clinically and pathologically mainly by light microscopy. Almost all the lesions have shown features which were predominantly those associated with either a plasma cell granuloma or a histiocytoma. Both conditions arise from a common chronic inflammatory origin and form one disease complex. In almost every instance the lesions show some features characteristic of the other. In two malignant change had occurred. The light microscopic and electron microscopic characteristics of this complex of lung lesions are described. A possible aetiological cause is suggested. The many names that have been given to this group of pulmonary lesions in the past have led to confusion and failure to recognize their inflammatory nature.