It is generally assumed that renal blood flow is symmetric in the absence of renal artery stenosis. The aim of the present study was to evaluate whether this is really the case. From a group of consecutive hypertensive patients who had undergone renal angiography, we selected those with patent renal arteries. In all of them selective renal blood flow (RBF) measurements (133Xenon washout technique) had been performed with blood sampling from aorta and both renal veins (n=148). Asymmetry of RBF, defined as > or =25% difference in RBF between left and right kidney, was present in 51% of the patients. Subjects with and without asymmetry did not differ in age, body mass index, blood pressure, creatinine clearance, renal volume, or activity of the renin-angiotensin system. The presence of asymmetry coincided with an increased rate of false-positive results on renal scintigraphy. Preliminary data suggest that there may be a relation between asymmetry and renal sympathetic nerve activity. This study demonstrates that asymmetry of RBF is a frequent finding in essential hypertension, which may confound the results of diagnostic tests for renal artery stenosis. Furthermore, the present results underscore the importance of studying the function of both kidneys separately, because it cannot be assumed that they are functionally equal.