We report two cases of renal vein thrombosis in renal transplants, both diagnosed by Doppler ultrasound. In both cases Doppler ultrasound showed complete absence of venous flow. In addition, the arterial signal showed a sharp systolic peak with a notch on the reverse diastolic component resembling an 'inverted M', a finding previously undescribed. This arterial waveform was present at both hilar and interlobar level. We postulate that this 'inverted M' sign may be more specific for renal vein thrombosis in that it may represent non-occlusive thrombus, despite the lack of detectable venous flow, whereas the previously described plateau-like reverse diastolic flow seen in some cases of renal vein thrombosis, also occurs in severe allograft rejection and is therefore a non-specific sign.