Mycophenolate mofetil (MMF) is a powerful immunosuppressive drug with established efficacy and safety. The search for a less expensive immunosuppressive protocol has led to an open randomised clinical trial of conversion from MMF to azathioprine (Aza). A total of 28 renal allograft recipients treated with prednisone, cyclosporine, and MMF was randomised into two groups: converted (early conversion) and control (late conversion). Conversion from MMF to Aza was conducted at the end of the 4th post-transplant month in the converted group and after the 12th month in the control. During the 20-month observation period, biopsy-proven acute rejection occurred more frequently in the converted than in the control group, although the difference was not statistically significant. Early conversion from MMF to Aza increased the risk of subsequent rejection in those patients who underwent at least one episode of acute rejection prior to conversion.