Fifty consecutive cystic renal masses in 43 patients were evaluated by both static scanning and real-time scanning techniques. Various parameters were evaluated and compared to ascertain whether the strict static scan cystic criteria were always present in comparable real-time evaluations. Four (8%) of the 50 masses were not classic cysts on real-time examinations, although the criteria were present on the static scans. Two had minimal acoustic enhancement and two had low-intensity internal echoes. Possible reasons for this difference were detailed and discussed. The potential problems in this observation were enumerated. Based on these observations, in certain cases when a cystic renal mass demonstrates poor acoustic enhancement or low-level internal echoes in real-time examination, static scanning may be helpful and may obviate the need for further studies. Indexing Words: Kidney Cyst * Real time * Mass Nephrosonography using conventional static articulated-arm imaging techniques (static scanning) has been shown to be extremely reliable for differentiating simple renal cysts from other cystic renal masses.1-4 As a result, a renal mass that meets strict ultrasonic criteria for a benign cyst need not be investigated further, but a mass that deviates from these criteria in any way requires further s t ~d y . ~ With the advent and current widespread popularity of high-resolution real-time ultrasonography, renal masses are often studied ultrasonographically solely with real-time techniques and the diagnosis of benignity is commonly made by invoking criteria that have been firmly established by static scanning. We have observed that a mass that is unequivocally a simple cyst by static scanning criteria will occasionally fail to meet the same rigid criteria when imaged with real-time techniques. In this situation, real-time imaging could result in some unreliable evaluations of cystic renal masses. Unnecessary further study and expense with additional imaging mo-