A method for measuring renal blood flow utilizing a dye curve recorded from the renal vein following the injection of indocyanine green into a renal artery has been evaluated in 29 kidneys of hypertensive patients. The early portion of the downslope of the dye curve could be described by a single exponential, which was distorted later by a changing slope that always corresponded to the expected time of arrival of recirculating dye. Renal blood flow calculated from the area under the extrapolated dye curve was compared in 11 kidneys with flow measured during ureteral catheterization by the PAH clearance method. Close agreement between the methods was noted in all but one kidney, in which the discrepancy was best explained as a deficiency in the PAH method. In kidneys supplied by a single renal artery, the smooth curves and exponential downslopes indicate that intrarenal mixing of dye is essentially complete. Apparently accurate flows were measured in those kidneys supplied by separate dorsal and ventral arteries, but slight irregularities in the curves suggest that complete venous mixing does not always occur. In two kidneys with accessory polar arteries, the dye was not adequately mixed and total renal blood flow could not be measured. Dye curves were utilized to assist in the evaluation of renal artery stenosis in 2 patients. The authenticity of a renal artery pressure gradient measured at the time of catheterization was established in 1 case by demonstrating that the catheter was not impeding renal blood flow. In a second case, a measured pressure gradient was shown to be at least partly factitious because of the prolonged dye curve and low flow obtained with the catheter advanced through the stenosis. It is suggested that the method may have clinical application for the measurement of separate renal blood flow, particularly when clearance methods cannot be performed. In addition, it offers the opportunity to study the effects of drugs and disease on renal transit time and renal vascular volume in man.