Hypothermia reduces ischemic damage if prolonged occlusion of the renal artery is required during an in situ kidney operation. Hypothermia may be achieved by external cooling or by transarterial hypothermic perfusion. External cooling may lower intrarenal temperature heterogeneously. Perfusion techniques via intra-arterial catheters introduced percutaneously are associated with minimal technical difficulties. The 95 patients who underwent extensive hypothermic nephrolithotomy had differential 131iodine hippuran clearance studies preoperatively, and 2 weeks and 6 to 46 months postoperatively. Sixty-three kidneys were cooled by transarterial hypothermic perfusion and 39 were cooled with topical ice slush. In the perfused kidneys renal function had decreased to 80.6 plus or minus 3.7 per cent (mean plus or minus standard error) of the preoperative value 2 weeks postoperatively but recovered to 92.1 plus or minus 4.6 per cent of the pre-treatment value. In the slush-cooled (topically hypothermic) kidneys renal function decreased to 69.7 plus or minus 5.1 per cent of the pre-treatment value 2 weeks postoperatively but recovered to 70.8 plus or minus 4.4 per cent of pre-treatment values 6 months or more postoperatively. Percutaneous transarterial hypothermic renal perfusion seems to offer better preservation of renal function in the clinical situation than does topical ice slush hypothermia.