PURPOSE The indications and the safety of non-ischemic partial nephrectomy using a microwave tissue coagulator were studied. MATERIALS AND METHODS Non-ischemic partial nephrectomy was performed on 17 kidneys of 16 patients using a microwave tissue coagulator. The diagnosis was renal tumor and renal stones in eleven and five patients, respectively. Renal tumors were less than 4 centimeters in diameter, while the stones were associated with a caliceal diverticulum or secondary cortical atrophy. Excision of the tumors was done via the retroperitoneal approach through an oblique lumbar incision. The needle of the microwave tissue coagulator was inserted around the tumor (stone) 10 to 20 times, and the coagulator was activated. Then the tumor (stone) was excised with a sharp knife or scissors. Patients were encouraged to walk on the first postoperative day. RESULTS Vascular clamping was necessary in one patient to reduce bleeding. Nephrectomy was done after partial nephrectomy in one patient because it was difficult to close the urinary collecting system after it was widely exposed. Although urine leakage was seen postoperatively in two patients, it ceased spontaneously at 14 and 23 days after surgery. Postoperative complications developed in one of seven patients (14%) with protruding renal tumor, in three of five patients (60%) with non-protruding renal tumor and in two patients with renal stone. Allogenic or autologous blood transfusion was not necessary, nor was any bleeding noticed post-operatively. In one patient, atrophy of the renal parenchyma occurred gradually after surgery and function was eventually lost. However, renal function was well preserved and recurrence of the problem was not observed in the other 15 patients, excluding one who died of esophageal cancer. CONCLUSIONS The microwave tissue coagulator is a useful surgical instrument for non-ischemic partial nephrectomy, not only in patients with renal tumors but also in patients with complicated kidney stones. However, non-protruding renal tumor in a patient with solitary kidney should be avoided for this surgery. Thermal injury to the renal parenchyma or large vessels should be avoided and urine leakage from the collecting system should be meticulously treated during the operation.