Background: Vitamin D plays a key role in the immune responses generated by lymphocytes and antigen-presenting cells. Decreased vitamin 25-hydroxyvitamin D (25(OH)D) levels have been implicated in several allergic disorders and association between 25(OH)D levels and chronic urticaria (CU) symptom scores has been evaluated in a few studies. This study was performed to assess the effects of vitamin D supplementation on the symptoms and quality of life scores in chronic spontaneous urticaria (CSU) and to vitamin D levels in CSU patients in comparison with controls. Patients and methods: Fifty-eight CSU patients and forty-five controls were included in the study. The patients were divided into two groups according to severity of the disease; as mild/moderate and severe urticaria. Serum 25-hydroxyvitamin D (25(OH)D) concentrations were measured in serum of CSU patients and compared with the control groups. In patients with 25(OH)D concentrations lower than 30 mu g/L, 300000IU/month of vitamin D3 supplementation was added to standard therapy. The clinical improvement was evaluated after 3 months with urticaria activity score (UAS4) and Chronic Urticaria Quality of Life Questionnaire (CU-Q2oL). Results: Serum 25(OH)D concentration was significantly lower in CSU group compared to healthy subjects (p<0.001). The prevalence of vitamin D deficiency (<20 (mu g/L) and insufficiency (<30 mu g/L) was significantly higher in CSU patients than control groups. In addition, 25(OH)D concentrations were significantly lower in both mild-moderate and severe CSU patients than those of the controls (p=0.011 and p<0.001, respectively). Ninety eight percent of patients (25(OH)D<30 mu g/L) were treated with vitamin D3 (300000IU/month) supplementation, and after 12 weeks, these patients showed significant improvements in UAS4 and CU-Q2oL scores. Conclusion: This study support the contributing and beneficial effects of vitamin D in the treatment of CU. Replacement of vitamin D may provide improvement in both the severity of symptoms and the quality of life scores in these patients.