Deterioration of renal transplant function due to rejection is accompanied by changes in renal blood flow that can be measured by duplex ultrasound (DU). In the present study, 60 transplanted patients were followed up. A total of 233 duplex examinations, 68 percutaneous biopsies, 44 renal scintigraphies, and 6 transplant nephrectomies were performed on these patients. Diagnoses were made on the basis of clinical and histological data. Renal perfusion was calculated by means of DU. In addition, the arterial Doppler signals were quantified using a pulsatility index (PI). Mean perfusion of normal renal allografts was 0.47 l/min. A decrease in blood flow could be found in acute vascular and chronic rejection but not in acute cellular rejection. A lack of renal perfusion due to acute vascular rejection was observed in four patients. Mean PI, used as a parameter of DU, rose significantly in all forms of rejection, which could be diagnosed with a sensitivity of 93% and a specificity of 86%. Cyclosporin overdosage did not alter the Doppler flow shape.