In a 71-year-old woman with primary squamous cell carcinoma of the thyroid, marked leukocytosis (26,000 to 87,000/mm3), in which about 90% were mature neutrophils, was observed. Hypercalcemia (about 12 mg/100 ml) was also observed with the serum inorganic phosphate within normal limits. The serum parathyroid hormone (PTH) was undetectable. The thyroid tumor was successfully transplanted to athymic nude mice at autopsy. Marked leukocytosis and hypercalcemia was reproduced in the tumor-bearing nude mice. The result indicates that the thyroid cancer was secreting humoral factors(s) producing leukocytosis and hypercalcemia. The association of leukocytosis and hypercalcemia has been reported in the two similar cases of squamous cell carcinoma, in which the tumor was proven to secrete colony-stimulating factor (CSF). Leukocytosis and hypercalcemia may thus form a new paraneoplastic syndrome.