The presence of multiple renal arteries per kidney is associated with hypertension. The smaller vessel diameter is thought to lead to decreased renal perfusion, which activates the renin-angiotensin-aldosterone system (RAAS), resulting in increased systemic blood pressure. We measured renal blood flow (RBF) using dynamic contrast enhanced (DCE) MRI to investigate the relation between number of renal arteries, RAAS activity and RBF. The number of renal arteries was associated with reduced RBF and increased RAAS activity. In all patients, we observed that reduced RBF was associated with increased RAAS activity.