Background/aim To measure the expression of 1α-hydroxylase (CYP27B1) and serum 25(OH)D concentration in systemic lupus erythematosus (SLE) and to investigate the role of CYP27B1 in SLE. Materials and methods Seventy-seven SLE patients and 35 healthy controls (HCs) were enrolled from September 2017 to January 2020. The study design is cross-sectional. mRNA expression of CYP27B1 in peripheral blood mononuclear cells (PBMCs) was measured by reverse-transcription quantitative PCR, the protein level of CYP27B1 was quantified by western blotting, and the serum level of 25(OH)D was determined by an enzyme-linked immunosorbent assay. Results The mRNA expression of CYP27B1 in PBMCs was significantly lower in SLE patients than in HCs ( p < 0.001), and the protein quantification confirmed that CYP27B1 expression was lower in SLE patients than in HCs ( p = 0.001). Among SLE patients, the prevalence of lupus nephritis was higher in a subgroup with lower CYP27B1 mRNA expression than in a subgroup with normal CYP27B1 mRNA expression (41.07% vs . 14.28%, p = 0.028). The mRNA expression of CYP27B1 negatively correlated with the Systemic Lupus Erythematosus Disease Activity Index ( r = −0.331, p = 0.003). Serum 25(OH)D concentration was lower in SLE patients than in HCs (37.64 ± 19.89 vs . 50.58 ± 12.74 ng/mL, mean ± SD, p = 0.003). Conclusion The expression of CYP27B1 in PBMCs may be related to SLE pathogenesis, disease activity, and nephritis.