We report a case of granulocyte colony-stimulating factor (G-CSF) producing lung cancer. A 38-year-old Japanese woman had a large cell undifferentiated carcinoma of the left lung with severe granulocytosis without any evidence of infection. A specimen was taken from a metastatic cervical lymph node and a tumor cell line was established. The culture supernatant of the line as well as patient's serum exhibited a high level of G-CSF by sandwich enzyme immunoassay. Immunohistochemical analysis demonstrated that tumor cells from transplanted nude mice were stained granularly in the cytoplasma by anti-human G-CSF monoclonal antibody, 4A6.