A 65-year-old was admitted to our hospital and was diagnosed as having squamous cell carcinoma originating in the right upper bronchus. He underwent both chemotherapy and radiation therapy, but these therapies were ineffective and thereafter the developed radiation pneumonitis and carcinomatous pleuritis. Finally, he died of bacterial pneumonia in the opposite normal lung of four months duration. From one month before his death, laboratory data indicated marked leukocytosis, and his granulocyte colony-stimulating factor (G-CSF) serum level was high. At autopsy, squamous cell carcinoma was found in the right hilus region of the lung, with a spreading form resembling a malignant pleural mesothelioma mainly occupying the pleural cavity. Based on positive staining method with specific monoclonal antibodies against G-CSF, it was considered that the leukocytosis was caused by G-CSF producing tumor.