Background: To evaluate the accuracy of the CoaguChek XS international normalized ratio (INR) monitor compared with the laboratory method in children with heart disease on anticoagulant therapy. Methods: Data on 120 pairs of INR values calculated from 42 patients younger than 16 years of age with heart disease were followed up at a cardiology clinic between May 1, 2007 and January 30, 2008. Parallel INR measurements were performed within one hour. The INRs from both the CoaguChek XS and laboratory tests were measured by a single qualified technician and were used to evaluate the accuracy of the two methods using linear regression and Bland–Altman analysis. Result: The mean INR was 1.93짹0.58 for the CoaguChek XS and 1.97짹0.60 for the laboratory test (P=0.63). In all INR parallel measurements, the difference between the two values was <0.5 INR units. The slope of the regression line was 0.99 (95% CI: 0.95–1.02) and the y-intercept was 0.014 (95% CI: –0.003–0.032). In the Bland–Altman analysis, the mean difference in INR between the two methods was 0.08 units, and the limit of agreement was –0.17-0.28 units. The values for 99.4% of all patients fell within the limit of agreement. Conclusion: INR measurement using the CoaguChek XS is as accurate but faster and more convenient than the laboratory method. Key Words: point-of-care, antithrombotic monitoring