A prospective nonrandomized study was conducted to evaluate the results of two conversion protocols on two similar groups of renal graft recipients totaling 54 patients who were converted from CsA to AZA at 6-12 months posttransplant. With protocol I, 24 patients (3 haploidentical, 21 cadaveric recipients) were converted with a graft biopsy followed by a 14-day overlap of CsA and AZA before the CsA dose was tapered and discontinued in 6 days. Of the 24 patients, 8 were found to have occult rejection in biopsy and received methylprednisolone 500 mg boluses for three days before overlap started. Thirty patients (2 haploidentical, 28 cadaveric recipients) were converted with protocol II, which had CsA and AZA overlap and tapering schedules identical to those of protocol I without a preconversion biopsy. Follow-up extended as far as 3 years posttransplant. There was a substantial incidence of chronic rejection and graft loss after conversion in protocol II patients. We also found that there was a possible link between postconversion acute rejection and late graft loss from chronic rejection. The incidence of acute rejection after conversion was significantly lower among protocol I patients as compared with that of protocol II (4% vs. 37%, P less than 0.001). However, if 8 patients with occult rejection in the preconversion biopsy were added to the total number of postconversion rejection in protocol I, the incidence of postconversion rejection in this group (38%) would be similar to that of protocol II. Using the time of conversion as the onset of the risk, protocol I patients had better graft survival than protocol II (100% vs. 80%, P less than 0.005) at 3 years posttransplant. If conversion becomes necessary, we recommend a preconversion graft biopsy to identify and treat patients with occult rejection before the beginning of CsA and AZA overlap, especially for those patients whose creatinine is higher than 2 mg/dl without obvious cause before conversion.