Paraneoplastic leukocytosis was defined as elevated white blood cell (WBC) levels caused by cytokines, likely produced by the tumor itself, without evidence of infection or myeloproliferative disease. We report a case of anaplastic thyroid carcinoma with leukocytosis caused by elevated production of granulocyte colony-stimulating factor (G-CSF) by the carcinoma. Initially, acute pyelonephritis (APN) was diagnosed and treatment for APN was ongoing, but the WBC count steadily increased to 68.8×10 9 /L. She was diagnosed with anaplastic thyroid carcinoma on her neck mass, and the serum concentration of G-CSF was found to be markedly increased at 1,010 pg/mL. In spite of supportive care, the patient's condition rapidly deteriorated and the patient died on day 23 of hospital stay. Leukocytosis without definite evidence of infection could be a paraneoplastic manifestation in patients with malignant tumors, and paraneoplastic leukocytosis may be related to poor prognosis.