A 73-year-old man was hospitalized because of weight loss and fever. Laboratory data showed marked leukocytosis (21,200/mm3), granulocytosis (89.7%), thrombocytosis (47.8 x 10(4)/mm3), increased CRP (15.8 mg/dl), and increased SCC (5.0 ng/ml). Chest X-ray films demonstrated a mass shadow in the right upper lung field. Chest computed tomographic scans revealed a mass shadow 58 mm in diameter with mediastinal pleural invasion in the right S1. Right upper lobectomy and dissection of regional lymph nodes was performed under a diagnosis of lung cancer (squamous cell carcinoma, T3 N0 M0 stage IIB) with concomitant infection. Serum G-CSF was 234 pg/ml pre-operatively and 68.8 pg/ml postoperatively. The cytoplasm of tumor cells stained positively with anti-recombinant human G-CSF monoclonal antibody. No general bacteria or mycobacteria were detected within the specimen. Postoperatively, the patient's white blood cell count, platelet count, and CRP level soon decreased, and the fever disappeared. We diagnosed the disease as G-CSF-producing squamous cell type lung cancer.