Early acute rejection occurs between two and ten days after renal homotransplantation and has been recognized in eight of 156 renal transplants. Four of the eight patients developed spontaneous renal rupture unrelated to biopsies. Pathologically, there was acute passive congestion with areas of tubular necrosis, venous thrombi and perivenous lymphoplasia. Glomerular and arterial immunogenic injury was slight. Radiologically, the kidneys were enlarged with stretching of the interlobar and segmental arteries, decreased peripheral filling and a homogeneous nephrogram. Renal vein filling defects were a frequent feature.