In 25 renal allografts out of 253 with graft dysfunction a reversed diastolic blood flow was observed at duplex ultrasonography during the first post-transplant month. Eleven grafts were lost and 14 survived. The cause of graft loss was arterial thrombosis (n = 1), venous thrombosis or obstruction (n = 4), steroid resistant acute rejection (n = 4) and acute tubular necrosis (ATN) (n = 2). The cause of graft dysfunction in the surviving grafts was ATN (n = 7) and acute allograft rejection (n = 7). Grafts with only peak-like or low velocity continuous diastolic flow reversal had a better prognosis (3 out of 15 were lost) than grafts with any other type of reversed flow pattern (8 out of 10 were lost). This difference was statistically significant (p < 0.01).