In 3 renal transplanted patients perirenal fibrosis developed within one year after transplantation. The kidney function slowly decreased and uremia and hypertension developed in spite of intensive immunosuppressive treatment. The characteristic morphological changes in kidneys with perirenal fibrosis were atrophy and fibrosis localized in the superficial cortex. At biopsy this condition could easily be separated from immunologic and infective lesions of the parenchyma, as well as from vascular complications causing decrease in kidney function and hypertension. The clinical histories indicate that the perirenal fibrosis was caused by complicating perinefritis and/or perirenal hemorrhage in all three cases. In rats a closely similar lesion could be experimentally produced. It is concluded that perirenal fibrosis should be considered as an alternative diagnosis to chronic rejection and vascular complications in transplanted patients with late graft failure and hypertension.