OBJECTIVE To assess renal functional deterioration after partial nephrectomy with warm and cold ischemia using (99m)Tc-mercaptoacetyltriglycine ((99m)Tc-MAG3) renal scintigraphy parameters. METHODS Open partial nephrectomy was performed in 59 patients with warm ischemia and 64 patients with cold ischemia. (99m)Tc-MAG3 renal scintigraphy was performed and effective renal plasma flow was calculated to evaluate split renal function. In addition, regional (99m)Tc-MAG3 uptake was analyzed in the surgically unaffected parts to evaluate ischemic damage. RESULTS The mean tumor size in the warm and cold ischemia groups was 2.9 and 3.2 cm, respectively, and the mean ischemic time was 24.2 minutes (range, 8-46 minutes) and 26.7 min (range, 8-58 minutes), respectively. One week after surgery, effective renal plasma flow in the operated kidney decreased to 66.2% (from 160.2 to 105.4 mL/min/1.73 m(2)) in the warm ischemia group and to 77.4% (from 152.3 to 116.6 mL/min/1.73 m(2)) in the cold ischemia group. Regional (99m)Tc-MAG3 uptake changed to 89.2% of baseline in the warm ischemia group and 101.5% of baseline in the cold ischemia group. When the ischemic time was ≥ 25 minutes, regional (99m)Tc-MAG3 uptake in the warm ischemia group did not recover to the baseline level at 6 months. Multiple regression analyses demonstrated a significant correlation between ischemic time and the decrease in regional (99m)Tc-MAG3 uptake in the warm ischemia group, but not in the cold ischemia group. CONCLUSIONS Warm ischemia for ≥ 25 minutes caused long lasting diffuse damage throughout the operated kidney, whereas cold ischemia for up to 58 minutes prevented ischemic injury to the renal remnant.