Background/aim: Vitamin D insufficiency is a common public health problem that is often unrecognized in children with congenital heart disease, and is not generally evaluated at congenital heart disease (CHD) follow-up. Recent studies have suggested that inadequate vitamin D status may have an adverse effect on cardiovascular health. This study investigates the relationship between vitamin D levels and hemodynamic parameters in children with CHD. Methods: Included in the study 40 patients (25 females, 15 males) with CHD, who were evaluated for Ross heart failure score, vitamin D, parathyroid hormone (PTH), calcium, phosphorus, alkaline phosphatase (ALP), whole blood count (WBC) and echocardiographic measurements, and all measurements were repeated in the third month of the therapy. Results: The mean vitamin D level was 16.4 ± 6.6 ug/L before and 27.5 ± 9.9 μg/L in the third month of therapy, while the mean PTH level was 53.3 ± 34.9 pg/mL before and 43.8 ± 21.4 pg/mL in the third month of therapy. The mean WBC was 8084 ± 2324/μL before and 7378±1893/μL in the third month of the therapy, and the mean platelet (PLT) count was 280,897 ± 80,119/μL before and 307,179 ± 60,202/μL in the third month of the therapy. The mean ejection fraction (EF) was 64% ± 7.2% before and 66.7% ± 6.2% in the third month of therapy, while the right ventricle (RV) myocardial performance index (MPI) was 32.1% ± 6.7% before and 28.9% ± 6.5% in the third month of the therapy. IL10 level was increased in four patients in the third month of therapy. A statistically significant decrease in PTH level and WBC, and an increase in PLT number and IL-10 level were detected by the therapy. Furthermore, echocardiographic findings revealed a statistically significant increase in EF and a decrease in RVMPI attributable to the therapy. Conclusion: The decreased levels of PTH, which is a proinflammatory marker, the increases in IL-10, which is an antiinflammatory cytokine, and the decreases in the number of WBC resulting from vitamin D treatment demonstrate the antiinflammatory effects of vitamin D. An improvement in EF means improvement in left ventricular contractility, while a decrease in RV MPI has been shown to improve the systolic and diastolic function of the right ventricle. These results suggest that vitamin D therapy has a positive effect on the heart, and so vitamin D levels should be evaluated during the routine follow-up of congenital heart disease.