Background: Preterm labor (PTL) is the most common cause of neonatal death worldwide. In Egypt, the average rate of preterm labor is estimated to be 14.7%. Objective: This study aimed to evaluate the relation between maternal vitamin D deficiency and idiopathic preterm birth. Patients and Methods: This case-control study was conducted on 160 pregnant women; 80 women with PTL as cases and another 80 women with term birth as controls, who were admitted to the Obstetrics and Gynecology Department in Ain Shams University Maternity Hospital. Results: Using ROC curve, serum 25(OH) D level ≤ 32 ng/ml was significantly associated with PTL (p = 0.001), with moderate sensitivity of 80% and low specificity of 47.5%. Conclusion: Preterm delivery was significantly associated with vitamin D ≤ 32.0 ng/ml. Although serum 25(OH) D deficiency is a significant risk factor, but it cannot be considered a diagnostic tool in predicting PTL.