OBJECTIVE To detect the serum level of mannose binding lectin (MBL) and its genovariation in systemic lupus erythematosus (SLE) patients and to investigate the role of MBL in the pathogenesis of SLE. METHODS ELISA was used to measure the serum MBL level of 40 SLE patients and 30 healthy blood donors. Tm genotyping method was used for the first timer in China. Primers and specific fluorophore-labelled hybridization probes for the exon 1 and promoter regions of MBL gene were designed based on the haplotype MBL2(*) LXPA (GenBank X15422). The genotyping of MBL in these two groups were performed using real-time PCR through Light Cycler Instrument. RESULTS (1) The serum MBL of the SLE patients was 107.2 microg/L, significantly lower than that of the healthy blood donors (290.2 microg/L, P = 0.0002). (2) MBL mutation in exon 1 region was mainly at codon 54, with a mutation rate of 37.1% in the SLE group, significantly higher than that of the control group (13.3%, p = 0.049). (3) Polymorphisms of H/L in MBL gene were present in both SLE patients and controls, and there was no difference in the L allele frequency between the two groups. (4) The serum MBL level of the SLE patients with MBL mutation in codon 54 was 49.8 microg/L, significantly lower than that of the SLE patients without MBL mutation in codon 54 (141.7 microg/L, P = 0.000 27). The SLE disease activity index (SLEDAI) of the SLE patients with MBL mutation in codon 54 was 7.44, significantly lower than that of the SLE patients without MBL mutation in codon 54 (12.87, P = 0.0029). A negative correlation was observed between SLEDAI score and serum MBL (r = -0.48). CONCLUSIONS Mutation occurring in MBL exon 1 region at codon 54 may be a predisposing factor of the pathogenesis of SLE. Serum MBL may be a potential biomarker of disease activity in SLE patients.