A total of 91 colour-coded duplex Doppler ultrasound scans were performed in the early phase following renal transplantation in 53 out of 109 patients (48.6%) between 1. 1. 1992 and 15. 3. 1993. The indications for the investigation were primary failure of functional improvement in the transplanted kidney or secondary worsening of function. In five of the 53 patients (ten scans) constant retrograde flow in the segmentary arteries provided specific evidence of renal vein thrombosis, this diagnosis being confirmed histologically following nephrectomy. The Pourcelot index was raised (RI > 0.72) in all five cases. The RI was also raised in 35 of 53 patients; however, continuous retrograde diastolic flow was absent. The results of renal biopsy and the clinical course both excluded renal vein thrombosis in these cases. The colour Doppler ultrasound scans were unremarkable in 13 patients. The comparison group consisted of 151 patients in the later posttransplantation phase (> 2 months), in whom 171 colour Doppler ultrasound scans showed no signals associated with renal vein thrombosis. In agreement with the literature, these results show that colour-coded duplex Doppler ultrasound should be regarded as the definitive diagnostic procedure for renal vein thrombosis after transplantation, and if positive findings are obtained further invasive diagnostic procedures can be avoided.