The relative importance of renal biopsy and cytological studies (fine needle aspiration biopsy and urine sediment examination by phase contrast) in the diagnosis of acute rejection episode was evaluated in 30 patients who received a kidney transplant. All patients underwent these 3 examinations simultaneously during an acute degradation of graft function. Sensitivity of renal biopsy, fine needle biopsy and urine sediment was 0.86, 0.68, 0.43 respectively. The association of fine needle biopsy and urine sediment examination increased the sensitivity up to 0.73. Renal biopsy is indicated to assess the vascular and glomerular status or in the presence of inconclusive cytological studies.