Background: Cardiac catheterization plays an important role in diagnosis and treatment of cases of congenital heart disease. Electrolytes are essential micronutrients that have important, physiological and metabolic roles in human being and cardiac functions.Objective: This study aims to evaluate the effect of cardiac catheterization either diagnostic or therapeutic on serum levels of selected electrolytes (sodium, potassium, calcium, magnesium and phosphorus) in cases of congenital heart disease.Methodology: This study enrolled upon 75 patients with congenital heart disease (cyanotic and acyanotic) aged from one day to 16 years. All patients were subjected to full clinical history, examination and specific cardiac investigations (echocardiography, ECG and chest x-ray), as well as detection of serum levels of sodium, potassium, calcium, magnesium and phosphorus before and after cardiac catheterization.Results: The results of the present study revealed that, all serum electrolytes’ levels decreased after cardiac catheterization. However, this decrease had no statistical significance except that of serum phosphorous.Regarding serum phosphorous, it was found that, there was a statistically significant decrease in the mean value of serum phosphorus level post-catheterization compared to pre-catheterization one in the studied group (p value = 0.010). Moreover, the current study revealed that, there is a highly significant statistical positive correlation between fluoroscopic time and DAP, with p value < 0.000.