We prospectively studied 551 sequential endomyocardial biopsies from 36 consecutive cardiac allografts. With the use of a combination of light microscopy (including careful evaluation of vascular changes) and immunofluorescence to detect the deposition of immunoglobulin and complement, we identified three patterns of allograft rejection, designated as cellular rejection, vascular (humoral) rejection, and mixed rejection. Cellular rejection was diagnosed with modified Billingham criteria. Vascular rejection was diagnosed by finding the combination of prominent endothelial cell swelling and/or vasculitis on light microscopy and the vascular deposition of immunoglobulin and complement by immunofluorescence. In such patients, cellular lymphoid infiltrates were uniformly absent at the time the vascular changes were detected. Mixed rejection consisted of findings of both cellular and vascular rejection occurring simultaneously. Twenty of 36 allografts exhibited cellular rejection; seven allografts showed vascular rejection, and nine allografts developed mixed rejection. The vascular (humoral) pattern of rejection was important to identify because the patients with this type of rejection had a significantly decreased survival compared with that of patients with cellular rejection (p less than 0.05). Survival in the mixed rejection category was intermediate. Positive donor-specific cross-match and/or panel-reactive antibody greater than or equal to 5% and systolic dysfunction were seen in three of the seven allografts with vascular (humoral) rejection but not in the other types. In the early period after transplant (up to 3 weeks after transplant), the only reliable identifying characteristics of patients with vascular (humoral) rejection were the presence of vascular immunoglobulin and complement assessed by immunofluorescence and endothelial cell swelling and interstitial edema as confirmed by histologic examination. We conclude that immunofluorescence should be routinely done on all heart biopsies for the first month after transplantation. Patients with vascular (humoral) rejection cannot be reliably identified by any other means.