The morphological abnormalities present in 570 endomyocardial biopsies from 39 cardiac allograft recipients and 16 autopsy hearts are described, and criteria pertinent to the diagnosis of rejection discussed. Entirely normal myocardium was apparent in 16% of the biopsies. Abnormalities related to biopsy of previous biopsy site occurred in 69%, and mononuclear infiltrates of varying intensity were present in 64% of the biopsies. Acute rejection was diagnosed in 17 (43.6%) patients in 32 (5.6%) of the biopsies and five hearts at autopsy. Seven of the biopsies with acute rejection were follow-up biopsies after a previous diagnosis of acute rejection and represented ongoing acute rejection. Similarly, three patient with rejection at autopsy died with ongoing rejection. The overall frequency of acute rejection was thus 0.70 episodes per patient. The most reliable histologic feature in the diagnosis of acute rejection in cyclosporine immunosuppressed recipients was a diffuse mononuclear infiltrate, apparent at low magnification. Myocyte necrosis, said to be critical in the diagnosis of rejection of cyclosporine-treated patients, was not a reliable indicator in the authors' experience.
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