Cyclosporin has improved graft survival after renal transplantation, but cyclosporin nephrotoxicity is a severe clinical problem. Conversion from cyclosporin to azathioprine 1 year after transplantation might improve long-term graft survival by avoidance of cyclosporin nephrotoxicity. After treatment with cyclosporin and prednisolone during the first year after renal transplantation, 106 patients were consecutively randomized to treatment with either azathioprine and prednisolone or cyclosporin and prednisolone in a prospective, controlled study during the following 5 years, i.e. 6 years after transplantation. Actuarial estimates of graft survival rates after inclusion in the study were obtained by the product-limit method of Kaplan-Meier, and the Mantel-Cox log rank test was used to compare the two treatment regimens. When the end-points in the analyses were cessation of graft function or withdrawal of immunosuppressive treatment due to side-effects, and when patients alive with graft function or who had died with a functioning graft were treated as censored observations, graft survival 5 years after inclusion in the study was 57.7 +/- 5.2% in the total material and was the same in both the azathioprine group (52.4 +/- 7.7%) and the cyclosporin group (63.3 +/- 6.7%) (log rank = 0.40, P = 0.53). When cessation of graft function was the only end-point, graft survival 5 years after inclusion in the study was 73.7 +/- 5.2% for the total material with no significant differences between the two groups (log rank = 0.58, P = 0.45).(ABSTRACT TRUNCATED AT 250 WORDS)