Graft-versus-host disease is well recognized in bone marrow transplantation, but has only recently been described in solid organ transplantation. Two such cases in liver graft recipients, proven by the demonstration of donor type HLA antigens in the peripheral blood and marrow on tissue typing, are described in this paper. The literature on this subject is reviewed and the treatment discussed. It is postulated that there is an order of risk of development of graft-versus-host disease depending on the amount of viable lymphoid tissue included with the transplanted organ as follows: small bowel greater than heart-lung greater than liver greater than kidney greater than heart. It seems likely that this condition has been substantially underdiagnosed in the past and that greater awareness of the possibility of graft-versus-host disease in solid organ recipients will lead to the recognition of further cases and allow appropriate treatment to be promptly instituted.