A total of 218 lung carcinomas from dogs and cats were examined histologically. The tumors were classified into adenocarcinoma, squamous-cell carcinoma, bronchial gland carcinoma, and alveolar-cell carcinoma. We believe that adenocarcinoma should be subdivided into differentiated and undifferentiated types because the two are distinct histologically and vary in frequency in the cat and dog. It is also important to recognize bronchial gland carcinoma, a distinct histological type, and to subdivide alveolar-cell carcinoma into three separate types: anaplastic small-cell and large-cell types, and adenomatosis type.