The histopathological changes in 51 renal allograft biopsies from patients immunosuppressed with FK506 were compared with those seen in 30 needle biopsies obtained from patients on cyclosporine. The frequency and severity of rejection episodes were similar in both groups. Tubular vacuolation and myocyte vacuolation were found to be useful morphological markers to monitor short-term drug toxicity associated with both drugs. Long-term administration of FK506 led to striped interstitial fibrosis and arteriolar hyalinosis, similar to that previously documented for cyclosporine. One case each of hemolytic uremic syndrome and necrotizing arteriopathy was noted in patients receiving FK506. FK506 and cyclosporine are structurally unrelated compounds; hence the parallelism observed in their nephrotoxicity profile suggests that the interactions of these drugs with renal tissue involves the operation of two different initial signal-transducing mechanisms, ultimately activating the same final metabolic pathways.