In 153 consecutive renal allograft recipients whose initial immunosuppression was prednisolone and azathioprine, 41 developed acute rejection episodes that were not reversed by 5-9 g of intravenous methylprednisolone. Renal histology showed cellular rejection in ten patients, vascular rejection in 12, and mixed cellular and vascular rejection in 16. Thirty-one patients were converted to cyclosporin in the first month post-transplant and ten in the second month. At the time of conversion, 20 patients were dialysis dependent and in the remainder the mean serum creatinine was 353 mumol/l (range 139-548 mumol/l). Renal function improved in 31 patients after conversion. Ten patients lost their grafts, of whom seven were on dialysis. There were no deaths and the 1-year graft survival was 75%. These data suggest that conversion from azathioprine to cyclosporin because of steroid-resistant rejection is an effective and safe strategy in patients whose initial immunosuppression is prednisolone and azathioprine.