A patient whose illness had begun with edema and hypertension was found to have extreme atrophy of both kidneys. Because of the steady worsening of the condition and the appearance of uremia with other unfavorable diagnostic signs, transplantation of 1 kidney from the patient's identical healthy twin brother was undertaken. Preparations included collection of evidence of monozygosity and experimental transplantation of a skin graft from the twin. During the transfer of the healthy kidney, it was totally ischemic for 82 minutes. Evidence of functional activity in the transplanted kidney was obtained. The hypertension persisted until the patient's diseased kidneys were both removed. The homograft has survived for 11 months, and the marked clinical improvement in the patient has included disappearance of the signs of malignant hypertension.