Background:Congenital heart diseases (CHD) are lesions caused by abnormal development of the structure of the heart.heart failure(HF) is a common complication of CHD.Elevated Fibroblast growth factor 23 (FGF-23) levels were associated with increased risk of cardiovascular diseases (CVD) and mortality in general population. Aim of the study: The aim of this study wasto assess serum FGF-23 levels among pediatric heart failure patients to demonstrate relationship between its levels and cardiac function and severity of the disease. Patients and methods: In our study we have evaluated 60 children. They were divided into two groups: A case group: Included 30 patient with HF secondary to CHD and A control group: Included 30 age and sex matched apparently healthy children.The diagnosis of cases was established from the history, clinical examination investigation. All patients were subjected to full history taking, a complete clinical examination including Ross Score for detection of the degree of heart failure, Complete blood count ,kidney function tests, chest X ray, ECHO was performed and serum level of FGF-23was also measured. Results:There were statistical significant decrease in Ejection fraction (EF) and Fractional Shortening (FS) of HF group compared to control group. There was significant increase in LVEDD, LVESD and ESPAP in HF group compared to control group. There was a statistical significant increase in serum FGF-23 level in HF group compared to healthy one. According to correlation analysis there was significant positive correlation between FGF-23 and Heart rate, respiratory rate, HB and Modified Ross score, also there was significant negative correlation between FGF-23 and FS, EF, while there was no significant correlation between FGF-23 and age, ESPAP, phosphorous and creatinine. The best cut-off value for FGF-23in HF patients was 141.5 ng/dl with sensitivity 88.3%, specificity 89.7%, positive predictive value 80.2%, negative predictive value 91.5%, accuracy 88% and area under curve 0.982. So, FGF-23 can be a good sensitive and specific marker for HF. Conclusion:This study demonstrated that FGF23 is diagnostic for heart failure with good sensitivity and specificity and related to severity of heart failure and correlate significantly with left ventricular function.