Abstract
In human lung cancers, marked leukocytosis, especially with neutrophilia and monocytosis, has been reported as a paraneoplastic syndrome. In dogs, several reports have been published on paraneoplastic leukocytosis. In this case, various examinations were conducted on a dog with extreme neutrophilic leukocytosis diagnosed via a blood test at first admission. A huge pulmonary mass was detected, which was surgically removed. The tumor was diagnosed as papillary adenocarcinoma of the lung by histopathology. The number of neutrophils recovered to normal after surgery, and the dog was diagnosed as paraneoplastic syndrome due to lung cancer.