Formal studies have not been published on the nephrotoxicity of FK 506 when the drug was used from the outset. This kind of information was sought in 101 recipients of primary livers, 24 hearts, and 3 double lungs or heart-lung. Perioperative renal dysfunction was commonly seen, which appeared to be related to FK 506 doses and plasma levels, particularly when the drug was given IV. This was reversible. Late renal function has been generally satisfactory in all three cohorts of patients, and the incidence of hypertension has been low. The therapeutic index of FK 506 is a good one, as revealed by these observations in patients whose most notable achievement was a low mortality.