Renal concentrating ability was tested in 20 severely injured patients who received an average of 21 blood transfusions. Timed measurements were made of urine output (UO), and the clearance of creatinine glomerular filtration rate (GFR), sodium (CNa), osmols (COsm), and free water (CH2O) during operation and were repeated at 12 and 18 hours postoperatively, and on postoperative days 2 and 4. During operation the GFR was markedly reduced (36 mL/min), while UO, CNa, and COsm were all markedly increased (8.5, 5.9, and 8.1 mL/min, respectively). The CH2O was positive (0.4 mL/min). Following operation the rate of renal excretion of water and solutes was still high: UO, 4.0 mL/min; CNa, 4.3 mL/min; and COsm, 6.0 mL/min. Five hours postoperatively the CH2O had returned to normal. By day 2 the excretion rate of water had returned closer to normal: UO, 2.1 mL/min; CNa, 2.6 mL/min; COsm, 4.0 mL/min; the CH2O was normal. Subsequent study results were normal. These data demonstrate a renal concentrating impairment during and following operation. An osmotic diuresis, transient tubular ischemia, a washout of interstitial inner medullary osmoles, or some cryptic factor may be causative.