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The aim of this study was to assess the significance of positive C4d capillary immunostaining of endomyocardial biopsies and its correlation to clinical outcome in adult heart transplant recipients.Nine hundred eighty-five endomyocardial biopsies from 107 heart transplant recipients were evaluated. Immunostaining for detection of intragraft C4d capillary deposition was performed on paraffin-embedded tissue using anti-human C4d polyclonal antibody.Positive staining of C4d was present in 36 patients (34%) and antibody-mediated rejection in eight patients (7%). The patients were subdivided into four groups on the basis of their C4d, circulating antidonor antibodies (donor-specific antibodies [DSAs]), and graft function: group 1=C4d positive, DSA negative, and no graft dysfunction; group 2=C4d positive, DSA positive, and no graft dysfunction; group 3=C4d positive, DSA positive, and signs of graft dysfunction, and group 0 (control)=all negative. An higher mortality risk was found in C4d-positive patients, when compared with negative ones (unadjusted hazard ratios: group 1: 18, group 2: 61, and group 3: 32-fold risk; P<0.0001).Antibody-mediated rejection is a complex and ongoing phenomenon with different phenotypic features. C4d positive predicts worse prognosis. C4d negative and DSA can be used as early mortality predictors in patients without signs of graft dysfunction.