An abnormal vascular status is present in the transplanted kidney. To define whether vascular factors might influence kidney function of the graft, the renal volume, blood flow and vascular resistance of a group of healthy subjects were compared with those of a group of well functioning renal transplants by color Doppler ultrasonography. Sixty healthy subjects and 75 well functioning cadaver renal transplant recipients were compared by color Doppler ultrasonography. Subsequently, 15 couples of donors and recipients of a living related renal graft were compared to observe the differences between the two organs of the same subject in a different environment. The variables studied were: the diameters and the volume of the kidney, renal blood flow and renal resistance index (RI). The group of cadaver renal transplant patients showed higher mean blood pressure (P = 0.009), higher serum creatinine levels (P = 0.0001) and lower endogenous creatinine clearance (P < 0.0001) than healthy controls. The length (P < 0.00001) and volume (P < 0.001) of the kidneys of cadaver transplanted patients were significantly greater than those of healthy subjects, while the length and volume of the living donors kidneys were identical to those of the recipients. RI, measured on renal vessels, showed lower values in healthy subjects and in kidney donors than in transplanted patients (P < 0.00001). Well functioning transplanted kidneys showed increased renal arterial RI. This non-immunologic factor did not appear to be detrimental with renal function in time, at least until 50 months after successful grafting.