A-73-year-old man, a heavy smoker, was admitted to our hospital for pain in the right lower quadrant of the abdomen. The patient had no complaint of respiratory symptoms. But he had noted a skin lesion on his right hip. Physical examination revealed mild tenderness in the right lower quadrant of the abdomen and a soft swelling on the right hip. Laboratory analysis revealed remarkable leukocytosis (38,600 mul: 86% neutrophils) and evaluated C-reactive protein (8.6 mg/dl). Chest radiograph revealed a mass shadow in the right upper field of the lung. A computed tomography of the chest on admission revealed a heterogeneous mass with mediastinal lymphadenopathy. Abdominal computed tomography revealed multiple metastases in the adrenal glands, gallbladder, intestine, and peritoneum. Fluorodeoxyglucose positron emission tomography demonstrated increased uptake in the peritoneal cavity and primary site of lung. There was no evidence of systemic infection. The histologic finding of poorly differentiated carcinoma was confirmed by a needle biopsy of the skin lesion on his right hip. His white blood cell count was elevated, 38,600 to 121,000 mul within 10 days. Suspecting a granulocyte colony-stimulating factor (G-CSF)-producing tumor, we measured serum G-CSF and subsequently found it to be elevated to 372 pg/ml. His condition was rapidly deteriorated, and he died of multiple organ failure 14 days after admission. The poorly differentiated carcinoma found at autopsy revealed positive immunoreactivity for G-CSF. There was evidence of multiple metastases in the adrenal glands, gallbladder, intestine, pancreas, liver, skin, and peritoneum. The final diagnosis obtained at autopsy demonstrated diffuse metastases spreading to peritoneal cavity resulting from G-CSF-producing lung cancer.