To evaluate the balance status of interleukin 18 (IL-18) and interleukin 18 binding protein (IL-18BP) in circulation in patients with lupus nephritis (LN) and primary nephrotic syndrome (PNS), plasma levels as well as mRNA expression in peripheral blood mononuclear cells (PBMCs) of IL-18 and IL-18BP were measured by ELISA and RT-PCR respectively. The ratio of IL-18/IL-18BP was also calculated. Both plasma IL-18 and IL-18BP increased significantly in LN patients while only IL-18BP increased in PNS, which resulted in an elevated ratio of IL-18/IL-18BP in LN but not in PNS patients when compared with normal controls. In contrast, increased level of IL-18 mRNA was only detected in LN but not in PNS group, although IL-18BP mRNA expressions in PBMCs in both groups were higher than that in control. The imbalance of IL-18 and IL-18BP might be involved in the pathogenesis of LN, based on which a therapeutic approach is valuable to be developed for LN. Cellular & Molecular Immunology. 2006;3(4):303-306.