Both magnetic resonance imaging (MRI) and ultrasound (US) have been reported to be useful in differentiating between acute allograft rejection and other causes of graft dysfunction in renal transplantation. The aim of this study was to evaluate the comparative usefulness of these techniques in the assessment of patients with acutely rising serum creatinine levels. Seventeen patients with 19 episodes of acute serum creatinine elevations were evaluated for the presence of acute rejection. The ultimate diagnoses of acute rejection were based on either renal pathological findings, or the response to standard antirejection therapy. Clinical, US and MRI diagnoses were assessed independently, without knowledge of the results of the other evaluation techniques. We found that US alone was useful in diagnosing acute rejection (x2 = 4.95, P less than 0.05), and when taken in the clinical setting was an added advantage (x2 = 6.68, P less than 0.01). MRI did not increase the diagnostic accuracy significantly.