Measurement of body temperature is the most common clinical test performed. The presence of fever is accepted as a reliable indicator of either acute of chronic disease. Although the vast majority of fevers are caused by infectious agents, some solid tumors have fever as a presenting illness usually because of associated inflammation or infection secondary to the neoplasm. However, cancer cells can spontaneously produce cytokines, small proteins with multiple biological properties. Some cytokines released by neoplastic cells are pyrogenic, ie, they produce fever directly by their action on the hypothalamic thermoregulatory center. Examples of malignant cells producing pyrogenic cytokines are renal carcinoma, lymphomas, and acute myelogenous and chronic myelogenous leukemias. The pyrogenic cytokines released by these cancers are interleukin-6, interleukin-1, tumor necrosis factor, and interferon, but others likely exist. Antipyretics reduce fever regardless of its causation; some studies suggest that fever caused by pyrogenic cytokines released from malignancies is preferentially reduced by nonsteroidal anti-inflammatory agents.