Endotheliitis (EN) is a feature of hepatic allograft rejection, characterized by the adherence of immunocytes to the endothelium of veins, often leading to endothelial damage, endophlebitis, and, sometimes, panphlebitis. We found EN at least once in 28 of 41 allografts (68%) that had survived 6 months or longer. In approximately half the affected cases, the condition recurred. The EN was mild in most instances; moderate or severe manifestations were found in only 13% of the cases. The histologic changes were present for about 1 week in approximately half the cases; a duration of more than 2 weeks was noted in 17%, and then EN usually persisted. After retransplantation, recurrence of EN was observed in all of nine cases. We were unable to establish specific clinicopathologic and laboratory correlations for EN. The immunocytes in EN consisted mainly of helper and suppressor/cytotoxic T cells as well as natural killer cells. Sometimes, the immunocytes assumed a blastlike appearance; in these instances, the condition was severe. Such blastlike changes may be specific for EN. The immunocytes were attached to the endothelium by pseudopodia, broad bases, or both; some also were interconnected by cytoplasmic bridges. Underlying endothelial cells often showed evidence of cytoplasmic damage. The pathogenesis of EN is not completely understood; the immunocytes probably attach themselves to antigenic epitopes. Their nature, however, has not been clearly identified; HLA-A, B, and C and HLA-DR were displayed in areas of EN, but the antigens also were found in vessels without EN.