OBJECTIVE The microwave coagulator is a useful instrument that enables surgeons to perform partial nephrectomy without vascular clamping. The extent of postoperative thermal damage in surgically spared renal tissue has not been well examined. The present study was conducted to evaluate the tissue damage caused by microwave coagulation in laparoscopic partial nephrectomy (LPN) for small renal tumors. METHODS Seven cases of LPN with a microwave tissue coagulator were entered in the present study. The median tumor diameter was 1.5 cm, and the median size of the resected specimen was 2.2 cm. Postoperative tissue damage was evaluated by contrast-enhanced CT 1 month after surgery. Surgically spared renal-tissue volume and functioning renal-tissue volume were estimated from the images by NIH Image 1.62 software. RESULTS Postoperative CT revealed unenhanced renal tissue adjacent to the surgical margin. The median estimated volumes of surgically spared and functioning renal tissue were 96.1% (range 74.3%-99.8%) and 88.4% (range 68.0%-92.7%) of preoperative normal renal tissue, respectively. The percentile volume of functioning to surgically spared renal tissue ranged from 89.9% to 96.0% (median 92.8%). CONCLUSIONS The microwave coagulator enables us to carry out partial nephrectomy without vascular clamping. Although 96% of normal renal tissue was surgically spared, 4% to 10% of this tissue was nonfunctioning as a result of microwave-induced thermal damage.