The pathogenesis of minimal change nephrotic syndrome (MCNS) is still not clear. Several studies indicate that MCNS is a systemic disorder of cell-mediated immunity. IL-12 is a pleiotropic cytokine that is produced primarily by macrophage and plays a primary role in the induction of Th1-mediated immunity. The purpose of this study is to explore the relationship of in vivo IL-12 levels in both sera and urine, and in vitro IL-12 production in stable or nephrotic clinical condition patients with MCNS. In vitro IL-12 production was detected from LPS-stimulated peripheral blood mononuclear cells (PBMNCs) of 20 MCNS patients during nephrotic and remission stages, levels of sera and urinary IL-12 were measured by commercially available ELISA kits (R & D Systems, Minneapolis). The results showed sera IL-12 levels and LPS-stimulated IL-12 productions for PBMNCs were significantly increased as compared with those of normal controls and in remission stage. However, there was also statistical difference of urinary IL-12 levels among MCNS patients during nephrotic and remission stage and controls. This study demonstrates a correlation between in vitro IL-12 release by PBMNCs and in vivo sera IL-12 level from MCNS patients as well as correlation with disease activity. The study contributes to the understanding of the pathogenesis of MCNS.