The diagnoses in 200 parallel fine-needle and core biopsies taken in acute renal allograft dysfunction, reduced function in long-term allografts, or in well-functioning grafts were compared. Fine-needle aspiration biopsy (FNAB) was found to be a reliable diagnostic tool with both a high sensitivity and specificity in acute cellular rejection (81 and 92%, respectively) and in normal kidney grafts (78 and 82%). On the other hand, the method was less valuable in the diagnosis of vascular rejection or interstitial fibrosis. Further evaluation is needed regarding the diagnostic implications of isometric vacuolization of tubular cells in FNAB specimens as a marker for acute cyclosporine nephrotoxicity.