The early and late nephrotoxicity of cyclosporine was examined in 23 orthotopic heart transplant recipients and two heart-lung transplant recipients up to four years after the operation. Cyclosporine was given orally pre-operatively and intravenously for the first two to five days after the operation. Subsequent daily oral therapy, in two divided doses, was adjusted to maintain serum trough levels in the range of 150 vg/ml to 200 vg/ml. By six months after the transplantation, the trough level was reduced to approximately 100 vg/ml. The mean pre-operative creatinine was 119 +/- 27 microM/L. After the operation there was transient oliguria in all patients, the mean peak creatinine (251 +/- 104 microM/L) occurring on days two to five. By day eight, creatinine (138 +/- 68 microM/L) was not significantly different from the pre-operative value. The maximal cyclosporine trough level (479 +/- 216 vg/ml) was also seen on days two to five and declined (195 +/- 53 vg/ml) on day ten. Despite the temporal association, linear regression analysis demonstrated no correlation between the early increase in creatinine and the cyclosporine level. There was, however, a correlation between the pre-operative creatinine and the peak post-operative creatinine. Two patients required dialysis but no death could be attributed to renal failure. By six months after the operation, serum creatinine had increased to 145 +/- 53 microM/ml and thereafter, with gradual reduction in cyclosporine dosage serum creatinine levels stabilized at 197 +/- 97 mu/mL at two years.(ABSTRACT TRUNCATED AT 250 WORDS)