A three-year horizontal comparison has been completed between national laboratories that certify specimens for the indirect verification of Charpy impact test machines. The participants in this study were the Institute for Reference Materials and Measurements of the European Commission, the National Metrology Institute of Japan (NMIJ), the National Institute of Standards and Technology in the United States, and the Laboratoire National d'Essais in France. The comparison was conducted to evaluate the impact of reference specimens over a three-year period. Sets of certified reference specimens, at low (15 J), medium (30 J), and high energy (100 J) levels were produced and distributed at the start of the study. Specimens were tested approximately every six months on each of the machines in the study. The results of the testing are presented and the stability of the various impact machines and specimens are discussed.