A 57-year-old man presented with jugular swelling. A leukocytosis of 50,000 WBC/μL (95% neutrophils) was found. PET/CT demonstrated increased fluorine-18-fluorodeoxyglucose uptake in the cervical/retrosternal mass, in the bone marrow, and in the enlarged spleen. Bone marrow biopsy confirmed myeloid hypercellularity with a left shift, but there were no signs of dysplasia and no evidence of tumor infiltration. Pathology of the thyroidectomy specimen confirmed an anaplastic thyroid carcinoma. Serum granulocyte-colony stimulating factor was elevated to 38.8 pg/mL (normal range, <0.6 pg/mL); granulocyte/macrophage-colony stimulating factor was normal (<2 pg/mL).