Cytokines have not been employed in clinical laboratory tests because of the many biological activities of individual cytokines and too complicated cytokine network. However, abnormal laboratory data and symptoms can be interpreted by blood cytokine levels. [Cytokines attributable to abnormal data and symptoms] For example, cytokines attributable to abnormal data and symptoms in rheumatoid arthritis are as follows: joint pain: TNFalpha, IL-1, IL-6, and IL-18; general fatigue and appetite loss: TNFalpha and IL-1; leukocytosis: G-CSF produced by IL-1-stimulated macrophages etc; thrombocytosis: megakaryocyte potentiating activity of IL-6; anemia: hepcidin up-regulated by IL-6, which inhibits iron absorption from the intestine, and IL-1, which decreases the blood iron level and promotes ferritin synthesis. [Differential diagnosis using blood cytokine levels] Blood cytokine levels are useful and important in the differential diagnosis of inflammatory disorders such as neutrophilia, eosinophilia, and especially in distinguishing tumoral fever from infectious fever in malignant lymphomas. [Disease/disorder-specific cytokines] In recent years, disease- or disorder specific cytokines have been identified, making cytokines more important in clinical use. For example, IL-18 for adult-onset Still disease; IFNgamma for hemophagocytic syndrome; IL-5 for allergic disorders; thrombopoietin for immune thrombocytopenic purpura; vascular endothelial growth factor for POEMS syndrome; PTH-rP for malignancy associated hypercalcemia. [Flow cytometric measurement of cytokines] Recently, a flow cytometric method has been developed in addition to ELISA. With this method, 30 cytokine concentrations can be measured simultaneously within four hours with a wide range of detection limit and high cost performance. Cytokines will be included in laboratory tests with this method.