Cyclosporine (CsA) and prednisone (Pred) are the mostly used drugs for immunosuppression after renal transplantation, but both drugs have marked side effects. Either replacement of CsA by azathioprine (Aza) or withdrawal of prednisone (Pred) resulting in CsA monotherapy can be employed to circumvent the adverse effects in the long run. Both treatment regimens were compared to this prospective randomized trial in patients who were treated with CsA and Pred during the first 3 months after renal transplantation (CsA: n=64, Aza-Pred: n=63, median duration of follow-up: 3.9 years). Estimated graft survival rates at 5 years after transplantation (in patients with a functioning graft at 3 months) were 78% in the CsA group and 87% in the Aza-Pred group. The incidence of a rejection within 3 months after start of steroid withdraw or conversion from CsA to Aza was 30% and 25% respectively (NS). At 2 years after transplantation, serum creatinine levels were lower in the Aza-Pred group (126+/-35 micromol/L) than in the CsA group (180+/-78 micromol/L; P>0.001). There were no differences in blood pressure or incidence of infections between the treatment groups. Treatment-related costs were measured during the first year after transplantation and were lower in the Aza-Pred group (DFL 40,882+/-18,895 vs. DFL 53,484+/-44,828; 1 DFL [Dutch guilder] is about US $0.60; P<0.005). In conclusion, CsA monotherapy and Aza-Pred treatment from 3 months after renal transplantation are comparably effective immunosuppressive treatment regimens, although Aza-Pred therapy results in better graft function. Withdrawal of steroids and replacement of CsA by Aza both carry a substantial risk of rejection. The previously demonstrated cost effectiveness of CsA-containing therapies seems to be limited to the first phase after transplantation. Conversion to Aza-Pred at 3 months after transplantation reduces costs.