Abstract Purpose A clinically manifested acute rejection is associated with graft dysfunction and with some ultrasound findings. The aim of our study was to determine the potential of ultrasound evaluation in the detection of subclinical acute rejective changes diagnosed in stable grafts by protocol biopsy. Methods Gray-scale evaluation, color Doppler imaging (CDI) and power Doppler imaging (PDI) was performed before each of 184 protocol graft biopsies in 77 patients in the third week, third month and first year after transplantation. The group was divided into four subgroups—normal histological finding, borderline changes, subclinical acute rejection of IA grade, and a clinically manifested acute rejection of IA grade. The sonographic findings were compared with individual groups. Results Detection of parenchymal edema using gray-scale imaging significantly differentiated borderline changes and subclinical acute rejection of IA grade from normal histological findings in the third week and in the third month ( P = 0.013, P = 0.002 and P = 0.024, P < 0.001), respectively. A similar finding could be recorded in the latter group in the first year after transplantation ( P = 0.024). The presence of edema and reduced peripheral parenchymal perfusion in PDI significantly more often indicated a clinically manifested acute IA rejection ( P = 0.019, P = 0.004, P = 0.044). Parenchymal CDI hyperperfusion had a high specificity (89.5%) but a low sensitivity (60%) in the detection of the subclinical form of acute IA rejection. Conclusion A composite gray-scale, PDI and CDI evaluation provide a significant differentiation of groups with borderline changes and subclinical acute rejection and groups with normal histological finding and clinically manifested acute rejection.