The pullback pressure from the left renal vein (LRV) to the inferior vena cava was studied in 16 patients with left renal bleeding of unknown origin (group A), 15 patients with hematuria of miscellaneous laterality (group B), and nine control subjects (group C). The mean pressure gradients were 5.0 +/- 2.0 mm Hg (mean +/- SD) in group A, 1.9 +/- 1.0 mm Hg in group B, and 1.1 +/- 0.9 mm Hg in group C. The mean pressure gradient of group A was significantly higher than that of groups B and C (P less than .001). From the results of control studies, we regarded pressure gradients greater than or equal to 3.0 mm Hg as indicative of LRV hypertension. With this criterion, 88% (14 of 16) of the patients in group A had LRV hypertension, which was considered a cause of hematuria. Analysis of the results of renal angiography in the 31 patients revealed that opacification of collateral pathways of the LRV was a significant angiographic finding in LRV hypertension.