A randomized trial was performed with the aim to compare two immunosuppressive treatment schedules in adult recipients of first cadaveric renal transplants. A total of 229 patients were randomized to double therapy with cyclosporine and prednisolone and 234 patients were randomized to triple therapy with cyclosporine, azathioprine, and prednisolone. Minimum follow-up was 4 years. The actuarial 5-year patient survival was 79.8% in the double therapy group and 82.3% in the triple therapy group (n.s.). The corresponding graft survival figures were 54.4% and 59.6% in the two groups, respectively (n.s.). There were no differences between the groups regarding cause of death or cause of graft loss. Renal function as determined by serum creatinine did not differ between the groups and was stable throughout the observation period. Azathioprine was instituted in a total of 51 patients randomized to double therapy. This subgroup of patients had a patient and graft survival not different from the remaining patients randomized to double therapy or from the patients randomized to triple therapy. There were no differences between the double and triple therapy groups regarding incidence and timing of acute rejection or infections. The incidence of other medical diseases and adverse events such as nephrotoxicity or malignancy did not differ between the groups. Azathioprine-induced leukopenia was uncommon (19 episodes in the triple therapy group). In a multivariate analysis of the whole series the only covariates that significantly influenced graft survival were age of recipient and occurrence of acute rejection, while among other factors treatment schedule did not. Thus this prospective study, in accordance with previous such studies, failed to find support for the use of triple therapy as first choice immunosuppression in first cadaveric renal transplantation. However, the study could not rule out the possibility that some patients at risk for the development of irreversible rejection or nephrotoxicity of CsA might benefit from the addition of azathioprine to the treatment schedule.