The high incidence of renal mass lesions, particularly the frequent occurrence of renal cysts in elderly patients, makes the differentiation of benign from malignant renal mass lesions an important clinical challenge. Accurate preoperative diagnosis is important for establishing the prognosis and for avoiding unnecessary surgery, with its increased morbidity and expense, when benignity can be established confidently. A broad spectrum of radiographically related imaging and biopsy techniques are currently available for the detection and characterization of renal mass lesions. In addition to plain film findings and radiographic contrast studies, nuclear medicine techniques and ultrasound analysis of renal mass lesions have made significant contribution to accurate differentiation of cysts from solid masses. Ultrasound scanning and cyst puncture in particular have attained critical significance in the workup of renal masses; Several recent publications analyze the accuracy, comparative costs, complications, and cumulative confidence of the various available procedures. An accuracy of 95% or greater appears attainable when definitive interpretation of the appropriate test is technically achieved, The current review will discuss the recent literature concerning renal mass lesions. The techniques and findings which should lead to acceptable confidence in the nonoperative differentiation of renal masses will be discussed. Factors regarding safety, accuracy, and expense will be considered.