Out of 81 liver transplantations 2 graft failures were diagnosed to be due to hyperacute rejection. In the first patient the operative procedure was difficult requiring 19 units of blood and plasma, but the graft was functioning well from the beginning until day 1-2, when rapid deterioration occurred. The cross-match was positive. The second patient received a third graft after the first graft had failed due to donor reasons and the second AB0-incompatible graft had been rejected. The third graft transplanted in an uncomplicated operation requiring only 10 units of blood and plasma failed within hours. Both incidences are thought to be a consequence of an immunological assault, consistent with hyperacute rejection. Thus two different clinical appearances could be observed: the so-called delayed type in the first patient and the more classical type in the second patient. For establishing diagnosis of hyperacute rejection two prerequisites were considered essential: 1) histological findings of necrosis and patchy deposits of immunoglobulins, namely IgG, IgM, IgA, C-3 complement component, properdine and fibrinogen, and 2) the proof of at least a short period of an initial function of the graft prior to deterioration in order to exclude primary non-function due to other causes. The low frequency of the appearance of the classical hyperacute rejection and hypothetical causes for the more frequent appearance of the delayed type are discussed.