PURPOSE: Intraoperative sonography was used to evaluate prospectively the renal parenchyma of patients undergoing surgery for presumed renal neoplasia. MATERIALS AND METHODS: One surgeon (F.F.M.) evaluated 100 kidneys using a triple-head sector ultrasonic transducer (5 MHz., 7.5 MHz. and 10 MHz. with duplex Doppler imaging). The size, number, echo texture, location, and relationship of suspected lesions to the collecting system, vasculature and renal capsule were recorded. Real-time Doppler imaging was used to distinguish among renal arteries, renal veins and the collecting system. Sonography was used to delineate the extent of the tumor and presence or absence of satellite lesions. All patients were evaluated preoperatively with abdominal computerized tomography. RESULTS: Under ultrasonic guidance partial nephrectomy was done in 40 cases, radical nephrectomy in 56, unroofing of renal cysts in 3 and renal biopsy in 1. Of the patients 8 were considered candidates for partial nephrectomy but underwent radical nephrectomy after intraoperative sonography revealed more extensive tumor, especially at the renal hilum. Similarly, 3 patients with a suspected malignancy were spared nephrectomy after intraoperative sonography and frozen section analysis revealed benign multilocular cysts. All surgical margins after sonographic evaluation were negative in patients undergoing partial nephrectomy. CONCLUSIONS: Intraoperative ultrasound is a useful adjunct for the dynamic evaluation of renal tumors in the surrounding environment of renal cysts, the collecting system and the renal vasculature. It is particularly beneficial in defining preoperative indeterminate renal lesions and in evaluating extrarenal structures for tumor involvement, such as the renal vein, inferior vena cava, adrenal gland and liver. Intraoperative sonography is most useful during partial nephrectomy and may improve tumor-free surgical margins. Sonography is routinely used during partial nephrectomy.