Introduction: The serum level of 25-hydroxyvitamin D deficiency has a role in the regulation of insulin secretion and suspected as a risk factor for gestational diabetes. The aim of this study was  to investigate the relation between 25-hydroxyvitamin D concentrations and gestational diabetes and to compare it with healthy pregnant women.
Subjects & Methods: A Case control study was conducted on 130 pregnant women (24-28 weeks) referred to Shahid Ghodsi health center in Mashhad (2011). First participants filled questionnaire forms including demographic data and obstetrics history, then Glucose challenge test was performed for them (GCT). In those with abnormal GCT, Glucose tolerance test (GTT) was performed. The subjects with abnormal GTT selected as case group and pregnant women who had normal GCT selected as control group. The concentrations of 25-hydroxyvitamin D, was measured in two groups. Data were analyzed by using SPSS software version 11.5, chi square test and T student test. P value less than 0.05% was considered as significant.
Results: Mean value of glucose of GCT was 158.68±22.70 mg/dl in case group and 106.13±22.48 mg/dl in control group and the difference between the two groups was significant (P<0.001). Vit D concentration in case group was 8.06±3.85 ng/dl and in control group was 17.75±5.38 ng/dl and the difference between the two groups was significant (P<0.001).  
Conclusion: Vitamin D deficiency is more prevalent in women with GDM than normal pregnant women and this is related to gestational diabetes.