Cardiac output (CO), renal blood flow (RBF), and glomerular filtration rate were measured in 29 young patients with borderline hypertension, in comparison with 26 normotensive controls of the same age and sex. In patients with borderline hypertension, both CO and RBF were significantly increased, whereas the RBF/CO ratio remained within the normal range. No significant changes in glomerular filtration rate and renal filtration fraction were observed. The study clearly demonstrated that renal ischemia was absent in young patients with borderline hypertension.