Six children (aged 1.3-6.9 years) were examined with serial duplex Doppler sonography and diethylenetriaminepenta-acetic acid (DTPA) isotope renography in the post-renal transplant period. The resistive index (RI) was derived from sonographic studies and the renal blood flow (RBF) calculated from the isotope scans. The clinical status of the child and the corresponding plasma creatinine level were assessed together with these two parameters. The RIs ranged from 40% to 100% and the RBF from 0% to 16.8%. There were six rejection episodes in four patients. A significant fall in RBF mirrored a rise in plasma creatinine on each occasion, but there was no significant change in RI recorded. There were two graft losses, both associated with renal venous thrombosis. In both cases no significant RBF could be detected on DTPA renography. In one patient, the RBF remained low throughout a period of primary non-function associated with acute tubular necrosis, and increased as primary function was established and the plasma creatinine fell. Throughout this period there was no significant change in the RI. From our preliminary data RBF reflects graft dysfunction more accurately than does the RI.