CoaguChek point-of-care testing is used widely for prothrombin time (PT)  testing in Germany and in The Netherlands and on an increasing scale in the United Kingdom, other European countries, and North America. CoaguChek monitors must give reliable displayed international normalized ratios (INR) within the 2.0-4.5 interval, because there is a dramatic increase in the risk of thrombotic and bleeding complications, respectively, at INR 2.0 and 4.5 (1). Recent reports have reiterated the need for reliable quality assessment (QA) of CoaguChek monitors (2, 3). Hitherto, no readily available system has existed for the local QA of CoaguChek monitors. Local QA is essential because International Sensitivity Index (ISI) calibration of individual monitors by the WHO ISI method (4) is not feasible. This is because of the need in ISI calibration for parallel conventional manual PT testing with the local PT test system (instrument/thromboplastin combination) and an international reference thromboplastin on plasma from the same whole blood samples used in tests on the CoaguChek. We report here on a joint study by the European Concerted Action on Thrombosis and the European Concerted Action on Anticoagulation (ECAA; now known as the European Action on Anticoagulation). The goal of this study was to assess the reliability of the European Community (EC)-approved QA method and compare it with conventional QA analysis.