Gray-scale ultrasound and biopsy were compared in 63 patients with suspected acute kidney allograft rejection. Sonographic findings of increased size and decreased echogenicity of renal pyramide, focal zones of sonolucency in renal cortex, and patchy sonolucent areas involving both cortex and medulla with coalescence correlated well with biopsy in 46 out of 50 patients (92%) treated for acute rejection (sensitivity = 0.92). Abnormal perirenal fluid collection was encountered in three out of 60 patients (5%). Out of 63 sonographic studies, 10 (16%) gave either false positive (6) or false negative (4) results for an overall accuracy of 85%.