Thalidomide was developed in the 1950s as a sedative drug and withdrawn in 1961 because of its teratogenic effects, but has been rediscovered as an immuno-modifying drug. It has been administered successfully for the treatment of erythema nodosum leprosum, aphthous ulceration in HIV disease, inflammatory bowel diseases, and multiple myeloma. So far, investigations into the mode of action of thalidomide have focused on lymphocytes and vascular endothelial cells and have shown that this agent inhibits the production of tumor necrosis factor (TNF)-α and is an inhibitor of tumor angiogenesis. Recently, other immunological effects of this drug have been gaining attention, including attenuation of neutrophil activation and inhibition of myelo-proliferative responses. In autoimmune diseases, inflammation is characterized by an influx of granulocytes, and the association of granulocytes with gastrointestinal ulcer formation or rheumatic arthritis has been well documented. The suppressive effect of thalidomide on the activation of the nuclear transcription factor NF-κB may explain these effects of thalidomide. NF-κB is retained in the cytoplasm with IκBα, and is activated by a wide variety of inflammatory stimuli including TNF, IL-1 and endotoxin followed by its translocation to the nucleus. Constitutive activation of NF-κB has been detected in various inflammatory diseases, while angiogenesis and organogenesis also require NF-κB activation. Thalidomide, on the other hand, has been shown to selectively suppress NF-κB activation induced by inflammatory mediators. NF-κB is known to be located downstream of proliferative and / or survival signaling induced by growth factors, which regulate anti-apoptotic genes. Myeloid cells in vitro, however, have been found to proceed to apoptosis as the result of the treatment with thalidomide and subsequent inactivation of NF-κB. These findings are consistent with clinical symptoms that showed the recovery from leukocytosis and / or neutrophilia after the administration of thalidomide. These findings shed new light on the anti-inflammatory properties of thalidomide and suggested that they may inhibit granulocyte-mediated tissue injury.