A 70-year-old man with small cell lung cancer was admitted to our hospital. He received chemotherapy consisting of cisplatin plus etoposide with concurrent chest irradiation. Because the patient had leukocytopenia after the first course of chemotherapy, he was treated subcutaneously with filgrastim (human recombinant granulocyte colony stimulating factor, G-CSF). Three days later, he developed psoriasiform skin eruptions mainly on the surface of the chest radiation field. When filgrastim was replaced with lenograstim (G-CSF), the skin lesions improved. But, after a second course of chemotherapy, lenograstim caused generalized psoriasiform eruptions. The patient had no previous history of psoriasis or any pre-existing skin disease. A skin biopsy revealed a Munro microabscess and spongiform pustules of Kogoj, which are the findings characteristic of the pathology of psoriasis. The MEDLINE report search has revealed, this is the first report of induction of psoriasis by G-CSF in a patient with small cell lung cancer.