Renal imaging studies (358) with quantitation of renal perfusion were performed following injection of 99mTc-DTPA in 22 children aged 6-15 yr with renal transplants. In 58 of 61 scans performed at the onset of a rejection episode, renal perfusion was decreased. This was the only feature of rejection in 6 episodes, in 5 of which the patient was oliguric due to post-operative acute tubular necrosis. In 4 episodes perfusion fell, but antirejection treatment was not given as a repeat scan was normal. On 18 occasions the plasma creatinine concentration (Pc) rose, but the scan was unchanged; no treatment was given and repeat Pc was normal. Three patients had a ureteric obstruction and 2 patients a lymphocoele detected from the images. There was no morbidity. Sequential renal scintigraphy was valuable in the early diagnosis of rejection and in the avoidance of unnecessary therapy.