Background: new Highly Sensitive Cardiac Troponin I assay (as hs-cTnI) represents an important advance with added sensitivity for cardiac myocyte necrosis and could have several distinct roles in clinical practice. Aim of the present study: it was to determine the serum level of highly sensitive cardiac troponin I (hs-cTnI) before and after pediatric cardiac catheterization (diagnostic or interventional), as a diagnostic biomarker of myocardial injury in these patients. Patients and Methods: a Prospective Cohort study was carried out during the period from January 2017 to June 2018 at Pediatric Cardiology Unit and Cardiology Department, Tanta University Hospital. It included 30 patients who were admitted and required pediatric cardiac catheterization. Doppler echo- cardiography and Estimation of serum highly sensitive cardiac troponin I (hs-cTnI): using the ELFA technique (Enzyme Linked Fluorescent Assay) was done. Results: there was highly significant increase of serum hs-cTnI levels immediately after and 12 hours after therapeutic pediatric cardiac catheterization (p <0.001), whereas in diagnostic pediatric cardiac catheterization there was no significant differences of serum hs-cTnI levels (p >0.05). There was significant negative correlation between serum hscTnI levels and Fractional shortening (FS) %, and also there was significant negative correlation between serum hs-cTnI levels and ejection fraction (EF) % (measured by echocardiography). Conclusion: serum highly sensitive cardiac troponin I (hs-cTnI) was significantly increased immediately after pediatric cardiac catheterization, with more significant increase after 12 hours, suggesting myocardial injury in these patients. Also, elevation of this diagnostic biomarker of myocardial injury was more after therapeutic than diagnostic pediatric cardiac catheterization.