Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) sensitise nerves to mechanical stimuli experimentally and may induce bone and muscle pain when used as supportive drugs. G-CSF and GM-CSF produced endogenously by tumour cells can cause paraneoplastic leucocytosis. Whether paraneoplastic leucocytosis is associated with changes in pain sensitivity is not yet clear. We report on a patient with advanced-stage thyroid cancer who developed extreme leucocytosis within a period of 4 weeks (103 000 white blood cells/mm(3)), composed mostly of neutrophils and eosinophils. Parallel to this leukemoid reaction, allodynia and hyperalgesia developed in the absence of tissue inflammation. The course of disease of an elderly male with advanced stage metastatic thyroid cancer with new onset neuropathic pain followed by the development of extreme leucocytosis in a leukemoid reaction suggests paraneoplastic release of myeloid CSFs. The coincidence of pain sensitisation and extreme leucocytosis suggests a causal contribution of G-CSF and GM-CSF.