Transplantation has become an accepted treatment of many cardiac end-stage diseases. Acute cellular rejection accounts for 15% to 20% of all graft failures. The first grading system of acute cellular rejection, the Stanford Classification, was introduced in 1979, and since then many other grading systems have evolved. Most recently, the International Grading System was introduced in The Journal of Heart and Lung Transplantation. In this study the interobserver reproducibility of both the Stanford Classification and the International Grading System is evaluated using Kappa statistics. Three observers evaluated 168 endomyocardial biopsy specimens according to the Stanford Classification and 100 endomyocardial biopsy specimens according to the International Grading System. The evaluation was carried out blindly. Kappa values of 54.1% and 51.5%, respectively, were obtained, both significantly above zero but not optimal. In addition to the interobserver reproducibility analysis of the two grading systems, the International Grading System is discussed. In the original description of the grading system terms such as focal, multifocal, and aggressive infiltrates and myocyte damage and myocyte necrosis are used. These terms create some difficulties in understanding or interpreting the various grades. The main problem is to distinguish between grade 1A and grade 3A. Despite the difficulties, the grading system is easy to use, but a revision is needed.