Real-time ultrasonographic diagnosis of renal agenesis may be difficult because of severe oligohydramnios. Recent reports have suggested that the absence of renal arteries on color Doppler ultrasonography may assist in the identification of renal agenesis. Because the position of the renal arteries is variable owing to fetal size, this study was undertaken to determine the location of the renal arteries as a function of fetal growth. Using color Doppler ultrasonography, 200 normal fetuses were studied in which the length of the aortic segment from the bifurcation of the iliac arteries to the renal arteries was measured. Regression analysis was performed in which the measured aortic segment was the dependent variable and the femur length the independent variable. Four cases of severe oligohydramnios were studied. The regression equation for the length of the aortic segment (Y) was significantly (P < 0.000001; R = 0.921) related to the femur length (X) (Yaortic segment (mm) = 3.1950 + 0.3034Xfemur length (mm); 99% prediction interval +/- 4.6 mm). In three cases of renal agenesis the renal arteries were not imaged with color Doppler sonography. Color Doppler imaging may be useful to identify the location of the renal arteries as they originate from the aorta. Using the data from this study, the location of the renal arteries can be determined for the corresponding femur length. If the renal arteries are not identified within the expected range (+/- 99% prediction interval) in fetuses in whom the kidneys are not identified with real-time ultrasonography, renal agenesis must be considered.