Vitamin D is a pleiotrophic hormone with immunoregulatory properties. Low levels of vitamin D have been discovered in various autoimmune diseases. Here, we investigated serum vitamin D levels in Koreans with systemic lupus erythematosus (SLE) and examined whether levels correlate with disease activity of SLE. Blood samples were prospectively collected from patients with SLE (n = 104) and normal controls (NC, n = 49) during the spring from March to May 2008. The level of serum 25-hydroxyvitamin D (25(OH)D3) was measured by radioimmunoassay. The serum 25(OH)D3 levels of patients with SLE (42.49 ± 15.08 ng/ml) were significantly lower than NC (52.72 ± 15.19 ng/ml, P < 0.001). Additionally, 17 patients with SLE (16.3%) had vitamin D insufficiency, while two NC had vitamin D insufficiency (4.1%). The risk of vitamin D insufficiency was 4.6-fold increased in SLE (P = 0.032). The serum 25(OH)D3 levels, adjusted with BMI, were positively correlated only with hemoglobin (β = 0.256, P = 0.018) and serum complement 3 (β = 0.365, P = 0.002). Serum vitamin D levels were lower, and vitamin D insufficiency was more common in Korean patients with SLE, however, our study demonstrated that vitamin D levels might not be a good marker of disease activity.