Four human cadaveric kidney homotransplants are described. The patients were treated with drugs intended to suppress theimmunological rejection processes. Three showed clinical evidence of rejection of the transplant at 15 to 19 days, apparently reversed by large doses of hydrocortisone-like steroids. All four developed further clinical evidence of rejection at 38 to 45days. Histologically the first rejection episode was associated with interstitial oedema and infiltration of the transplant by lymphoid and pyroninophilic cells. The second episode was characterized by marked intimal thickening and medial changes throughout the whole renal arterial tree leading to obliteration of many vessels and ischaemia of the transplant.