Background: Systemic lupus erythematosus (SLE) is a chronic multisystem autoimmune disease. Vitamin D has a modulating effect on immune responses. Hypovitaminosis D is highly prevalent in SLE patients, and it may lead to SLE activity and SLE-related neuropathy. Aim of the study: To recognize the role of serum vitamin D levels in SLE activity and also to investigate its relation to SLE-related neuropathy. Patients and Methods: the current study was a cross-sectional study performed on 100 SLE patients, who were divided into two groups, Group 1: included 50 patients with disease activity. Group II: included 50 patients without disease activity. They were tested for serum vitamin D levels, serum electrolytes, complement levels and nerve conduction. Results: Vitamin D was significantly low in group1 (median = 9.0 ng/ml) compared to the group 2 (median = 19.3 ng/ml and P-value of<0.001). Hypovitaminosis D was statistically significantly correlated with lower levels of complement (both C3 and C4) in the activity group but not in the non-activity group. Vitamin D levels were significantly associated with delayed nerve conduction in both groups, suggesting that neuropathy was linked to vitamin D level rather than SLE activity Conclusion: Hypovitaminosis D is statistically significantly correlated with SLE activity and SLE-related neuropathy.