An increased level of serum cardiac troponin I is a specific biomarker for myocardial injury. The objective of this study was to evaluate the changes in cardiac troponin I levels to determine myocardial injury following diagnostic cardiac catheterization. A prospective cohort study was carried out between July 1 2006 and March 31 2012 among patients who underwent diagnostic cardiac catheterization in a tertiary hospital. Blood samples were collected at baseline and at 6- and 12- hours of the procedure to measure any changes in cardiac troponin I levels. A total of 264 adult patients; consisting of 157 males and 107 females, who underwent diagnostic cardiac catheterization were chosen for the study. Out of them 42 (16%) had a positive family history of coronary artery disease (CAD). Before the procedure, the mean cardiac troponin I level was 0.059 ± 0.030 ng/mL. However, the mean cardiac troponin I level after 6 hours of catheterization was 0.062 ± 0.041 ng/mL (P = 0.189) and was 0.063 ± 0.026 ng/mL (P = 0.099) after 12 hours. Average cardiac troponin I levels at 6- and 12- hours following diagnostic cardiac catheterization did not differ according to patients’ demographic or clinical characteristics (P >0.05). No in-cathlab complications and major adverse cardiac events were observed after onemonth. There were no significant changes in cardiac troponin I levels before or at 6- and 12- hours after diagnostic cardiac catheterization. This study, therefore, suggests that diagnostic cardiac catheterization does not appear to be associated with substantial subclinical myocardial injury.