PYELOVASCULAR backflow has been observed since the earliest days of urography. Little understood, it has been the basis for much controversy. Little clinical significance has been attributed to it. We have repeated and added to earlier work done in the investigation of pyelovascular backflow. We have been able to quantitate the levels at which pyelovascular vs. pyelotubular backflow occurs, and we have demonstrated that pyelorenal backflow may have considerable clinical significance. The mechanism of pyelovascular backflow has not been determined. We have demonstrated such a mechanism by correlating the histopathology and intrapelvic pressure with microradiology. Review of Literature Hinman and Lee-Brown in 1924 reported a series of studies relating pyelotubular and pyelovascular backflow to pressure (2, 3). They measured the secretory pressure of a dog kidney at 70–90 mm Hg. With Bird and Moist they noted pyelotubular backflow with gradual increase in pressure. Bird and Moist were unable to produce pyelo...