Posttransplant diabetes mellitus (PTDM) occurs in approximately 15% to 20% of renal transplant patients. It has important clinical implications for graft function and survival. Anticalcineurin drugs are associated with an increased risk of developing PTDM. There is a little evidence that conversion from tacrolimus to cyclosporine (CsA)-based immunosuppression improves glucose metabolism and reverses diabetes. This prospective study included nine renal transplant patients (mean age of 34 +/- 20) with PTDM under immunosuppression with tacrolimus. Five were switched directly to CsA and the other four (glycemia > 250 mg/dL) required insulin and were simultaneously switched to CsA. Basal blood levels of tacrolimus were 7.9 +/- 1.9 ng/dL. Conversion was associated with an early, significant improvement of glycemia and HbA1c blood levels (P < .01). At the end of the follow-up, the glycemia (105 +/- 20 mg/dL) and Hb1Ac (5.1 +/- 0.4 mg/dL) were normal. Insulin was discontinued between 3 and 6 months in all patients who required it at the beginning. Cholesterol did not change significantly and triglycerides decreased significantly (basal 210 +/- 85 mg/dL, at 12 months 125 +/- 29, P < .01). Graft function was stable with a mean serum creatinine of 1.7 +/- 0.2 mg/dL. CsA blood levels remained stable during all follow-up periods (P = NS). There were neither episodes of acute rejection nor secondary effects related to the medication. In summary, renal transplant patients receiving tacrolimus who develop PTDM may display better control of hyperglycemia by a switch to CsA.