Although autopsy studies have demonstrated valves in 28-70% of right and 4-36% of left renal veins, they have rarely been detected or described during life. In an analysis of 98 renal venograms, valves were found in 16% of patients on the right side and in 15% on the left, without predilection for any particular location. Angiographically, they appeared as thin, web-like structures which may block passage of the catheter or of contrast material, and hence cause poor venographic filling. Rarely, valves produce total obstruction to the retrograde flow of contrast material.