Alloantibody frequently appears during the immune response to alloantigens in renal transplant recipients. We studied whether the presence of antibody against donor class I antigens correlated with the clinical and pathologic features of acute rejection episodes. We identified patients who had (1) clinical evidence of acute rejection, (2) a renal biopsy showing pathologic features of acute rejection, defined by the Banff criteria, and (3) pre- and posttransplant sera screened against donor T cells. We divided these patients into those with or without donor-specific alloantibody reactive with donor T cells. Of 44 patients with biopsy-proven rejection, 20 were antibody negative (Ab-R) and 24 were antibody positive (Ab+R). The biopsies from Ab+R patients had a higher incidence of severe vasculitis (P=0.0009) and glomerulitis (P=0.01). Fibrin thrombi in the glomeruli and/or vessels, fibrinoid necrosis, and dilatation of peritubular capillaries were also more frequent in the Ab+R group. Infarction was present in biopsy specimens from 9/24 Ab+R patients versus none in the Ab-R group (P=0.002). The Ab+R biopsy specimens more often had polymorphonuclear leukocytes in the peritubular capillaries (P=0.003). In contrast, specimens of Ab-R patients showed tubulitis more often than the specimens of Ab+R patients: moderate and severe tubulitis was present in 19/20 (95%) Ab-R patients versus 12/24 (50%) Ab+R patients (P=0.002). Graft loss was increased in Ab+R patients, particularly in the first 3 months (12/24 compared with 3/20, P=0.025). Thus, during biopsy-proven acute rejection episodes, anti-class I antibody correlates with severe vascular lesions, glomerulitis, and infarction, whereas more severe tubulitis predominates in rejection episodes without antibody.