The aim of this study was to investigate the regulation of interleukin (IL)-12 and IL-23 expression in the autoimmune disease, systemic lupus erythematosus (SLE). mRNA from healthy subjects and SLE patients were prepared from peripheral blood mononuclear cells (PBMC) and quantitative real-time polymerase chain reaction was performed to quantify IL-23 specific subunit P19, IL-12 specific subunit P35, and their common subunit P40. IL-12 specific subunit P35 mRNA expression in untreated and treated SLE patients was significantly lower than healthy controls (P = 0.015 and 0.000, respectively). Compared with untreated SLE patients, treatment of SLE patients with corticosteroids or corticosteroids plus another immunosuppressor significantly suppressed P40 and P19 expression (P = 0.002 and 0.015, respectively). The mRNA levels of p19, p40, and p35 in active SLE patients (SLEDAT > 10) were significantly higher compared with those in the inactive SLE patients (SLEDAI <or= 10) (P = 0.000, 0.000, and 0.017, respectively). These results suggest that deficiency of IL-12 and possibly upregulation of IL-23 may contribute to SLE pathogenesis and both cytokines may be therapeutic targets in SLE.