Usefulness of the real-time two-dimensional Doppler scan system for the detection and measurement of blood flow of the allografted kidney was investigated in 6 patients with renal transplantation. Directions of the blood flow of both artery and vein of the allografted kidney were readily recognized in color displays using the color-Doppler. A velocity profile of the arterial blood flow could be measured using the pulse-Doppler without difficulty. The maximal arterial flow velocity of the patients with functioning allografted kidneys ranged from 25 to 132 cm/sec. In contrast, the arterial velocity profile of a nonfunctioning allografted kidney could not be measured in spite of the vissible direction of the blood flow in color. In conclusion, the real-time two-dimensional Doppler scan system is valuable in detecting and measuring the blood flow in patients with renal transplantation. Therefore, it may be suspected that this method will become a new hemodynamic examination for the allografted kidney.