Fine-needle aspiration biopsy (FNAB) was performed in 41 renal transplant patients to assess its value in the diagnosis of the cause of graft dysfunction. The procedure was used on 71 occasions, and in each case a clinical diagnosis was made and compared with the cytological diagnosis made independently by a pathologist. When available, core biopsy histopathology was used to confirm the final diagnosis. Fifty-seven (80%) of the aspirates yielded adequate material. A clinical diagnosis of acute cellular rejection (ACR) was made in 21 patients, 18 (78%) of whom showed confirmatory immune activation on FNAB. The clinical diagnosis of either acute tubular necrosis (ATN) or cyclosporine toxicity was confirmed in 31 (91%) of 34 aspirates. In eight aspirates, the cytological diagnosis was different than that made clinically. Humorally mediated vascular rejection, and lymphocytosis secondary to causes other than rejection, such as viral infection, were considered as possible causes of this discrepancy. Provided that adequate samples are obtained. FNAB is valuable in the clinical management of renal transplant patients. Its accuracy should not be overestimated and the results obtained should be evaluated in the light of the overall clinical picture.