Interleukin-6 (IL-6) is a pleiotropic cytokine with various biological activities. Deregulated overproduction of IL-6 has been found to play pathological roles in chronic inflammatory diseases such as rheumatoid arthritis, Castleman's disease, juvenile idiopathic arthritis and Crohn's disease. Humanized anti-IL-6 receptor antibody has been developed as a therapeutic agent for these diseases, and therapeutic benefits have been revealed in clinical studies.