Cancers associated with marked neutrophilia are relatively rare. We report here two cases of anaplastic thyroid carcinoma associated with neutrophilia. We measured the concentrations of granulocyte colony-stimulating factor (G-CSF), macrophage CSF (M-CSF), granulocyte-macrophage CSF (GM-CSF), interleukin-1alpha (IL-1alpha), IL-1beta, IL-6, and tumor necrosis factor-alpha (TNF-alpha) in sera, pleural effusion, cyst fluid of the thyroid carcinoma region, or culture supernatants of carcinoma cells. Maximum levels of elevated white blood cell counts reached 106.1 x 10(9)/L (neutrophils 103.0 x 10(9)/L) in case 1 and 62.3 x 10(9)/L (neutrophils 57.9 x 10(9)/L) in case 2. Acute-phase reactants were elevated to various degrees, and hypercalcemia was found in both cases. IL-6, G-CSF, and M-CSF seemed to play the principal roles in neutrophilia in case 1, and the elevated levels of IL-6 and M-CSF seemed to mainly contribute to neutrophilia in case 2. Immunohistochemical staining revealed that carcinoma cells themselves produce IL-6 regardless of the types of carcinoma cells. To our knowledge, this is the first report describing the contribution of M-CSF to neutrophilia in patients with thyroid carcinoma.