Objective To investigate the serum concentration of soluble klotho (s-kl) in systemic lupus erythematosus (SLE) and lupus nephritis (LN) ,and elucidate its role in SLE and LN .Methods A total of 34 patients ,definitely diagnosis as SLE with un-treatment firstly ,were enrolled in this study .The patient were divided into two groups ,those who complicated with LN were assigned to LN group (15 cases) ,the others were distributed to SLE group (19 cases) .At the same time ,17 cases of routine physical examination people were take as control group .24 hours urine of all the cases was collected for examining urinary protein (Upro) .Routine hemocyte analysis ,serum biochemical parameters and ANA and dsDNA were measured by routine method .ELISA was used to detect s-kl ,25-hydroxy vitamin D (25-OH-D) and fibroblast grow th factor-23 (FGF-23) .Systemic lupus erythematosus disease activity index (SLEDAI) was performed in SLE and LN group ,and the creatinine clearance rate(CCr)was calculated accord-ing to the Cockcroft-Gault formula .Pearson′s and linear regression were applied to analyisis the correlation of relevant parameters . Results As compared with the control group ,there were statistically significant differences in mean arterial pressure ,blood rou-tine ,creatine kinase (CK) and lactate dehydrogenase (LDH ) ,complement (C3 and C4 ) and dsDNA in SLE and LN group (P <0 .05) .The level of Upro/24 h ,lipids (CHOL and TG) and creatinine (Scr) in LN group was significantly higher ,while serum al-bumin (ALB) and CCr were obviously lower than those in SLE group and control goup(P< 0 .05) .Difference of s-kl ,25-OH-D and FGF-23 in serum were not observed between SLE and control gropu(P> 0 .05) ,but the serum level of s-kl ,25-OH-D and FGF-23 in LN group were showed a statistical significance when compared with SLE or control group(P< 0 .05) .Meanwhile ,the SLEDAI score was higher in LN than in SLE group(P< 0 .05) .Correlation analysis indicted that s-kl exhibited a positive relationship with 25-OH-D ,C3 and C4 ,while showed a negative correlation with FGF-23 ,SLEDAI and dsDNA(all P< 0 .05) .However ,no any corre-lationt was revealed in regression analysis between the s-kl ,25-OH-D ,FGF-23 and the lupus activity .Conclusion The decrease of s-kl maybe one of the pivotal factors that up-regulated the level of FGF-23 in SLE and LN patients ,thus lead to the deficiency of vi-tamin D and lupus activity .