A brief description is given of the most important morphological changes in endomyocardial biopsy specimens taken from cyclosporin A (CyA)‐treated cardiac allograft recipients. The National Hospital of Norway was, in 1983, the first Scandinavian hospital to perform a heart transplantation, an event facilitated by the new immunosuppressive drug CyA. Up to now (February 1987), 38 allogeneic orthotopic transplants have been carried out on a total of 37 patients, of whom 32 are alive and clinically well. There was no operative mortality, but 5 patients died of other causes: Two early rejections, one arrhythmia due to moderate rejection, one Toxoplasma myocarditis and one early graft failure due to donor heart coronary artery disease. The observation time ranges from six weeks to 39 months. The first 9 patients received CyA and prednisolene; all subsequent recipients were treated with Azathioprine additionally. Thirty six grafts were controlled by 557 sequential biopsy prosedures which yielded 2783 endomyocardial specimens for histopathological examinition. A histological diagnosis of rejection was made 99 times in 32 grafts (mean 2,6). Twenty four biopsies were obtained on clinical indication in 15 patients, and rejection was diagnosed in 11 biopsies. Evaluation of endomyocardial biopsies is important in monitoring cardiac recipients and provides a morphological index of acute rejection. Serial biopsies with adequate endomyocardial sampling from different areas of the right ventricle make it possible to diagnose acute cellular rejection at an early stage and are essential to control immunosuppressive treatment. Endomyocardial biopsy is a safe and reliable procedure and plays an important role in the management of cardiac allograft rejectio