AbstractDilated cardiomyopathy (DCM) is the third leading cause of heart failure in pediatrics. The exact etiology of DCM is unknown in more than half of the cases. Vitamin D receptors are represented in cardiac muscles, endothelium, and smooth muscles of blood vessels suggesting that vitamin D could have a vital cardioprotective function. This study aimed to assess serum level of vitamin D in children with idiopathic DCM and to correlate the serum level of vitamin D with the left ventricular dimensions and function. This study is a descriptive cross-sectional single-center study, includes 44 children of both sexes, diagnosed as idiopathic DCM. Serum level of vitamin D was assessed and correlated with the left ventricular dimensions and function. Mean age of studied children was 6.08 ± 4.4 years. Vitamin D deficiency was found in 90.9% of children with idiopathic DCM with a mean level 13.48 ng/mL. There was a negative correlation between vitamin D level and fraction shortening and left ventricular end-diastolic diameter in children with DCM. Vitamin D level is not only significantly low in children with idiopathic DCM but it is also significantly correlated with the degree of left ventricular dysfunction.