In six of twelve orthotopic liver recipients nephrotoxicity was noted after 13-22 days of treatment with 16.3 + or - 2.9 (SEM) mg/kg per day of cyclosporin A (CyA). With a decrease in the daily CyA dose to 9.2 + or - 2.3 (SEM) mg/kg kidney function returned to normal. No hepatic rejections occurred on this lowered CyA dose. In 4 out of 66 kidney recipients a switch from a CyA dose of 5.2 - 10.7 mg/kg daily to azathioprine was done 4 - 8 months after transplant because of unsatisfactory kidney function, suspected to be due to nephrotoxicity. In three patients, this resulted in an improved graft function. A fourth transplant was lost to an irreversible rejection 13 days later. Thus CyA is nephrotoxic but this toxicity is easily reversed, even after many months of treatment, and the ease with which this complication can be managed suggests that nephrotoxicity should not diminish the high expectations that transplant surgeons have for CyA.