The author reconsiders Stanford classification which separates 3 degrees of rejection of transplanted hearts: minimal, moderate or severe rejection. This classification, widely used, was enriched with J. Kennitz's sub-varieties, based on more than 4,000 endomyocardial biopsies, performed on 132 patients who underwent heart transplantation. Interpretation of the histopathologic results of endomyocardial biopsies requires careful knowledge of simultaneous clinical or biochemical data. However, heart biopsy remains indispensible since it is the only test providing information on the presence or absence of cellular infiltrate and on myocardial fibers changes. The difficulties result from the very small size of all samples. Several types of rejection are illustrated and commented. However, the author's experience is presently restricted and hardly allows separating moderate rejections from minimal or severe rejections.