This pilot study was designed to evaluate the safety and efficacy of converting from a calcineurin inhibitor (CI) to a sirolimus (SRL)-based regimen in established renal transplant recipients with moderate renal insufficiency. Sixty renal transplant recipients on CI-based immuno-suppression with a serum creatinine (SCr) between 159 and 265 microM (1.8 and 3.0 mg/dL) and a glomerular filtration rate (GFR) between 30 and 70 mL/min were enrolled. SRL dosing was dependent upon concomitant immunosuppressive therapy. The mean patient age was 45 yr and the mean time from transplant to study enrollment was 60.8 months (range: 7-198). The median SCr was 168 microM (1.9 mg/dL) and the median GFR was 51 mL/min. Twelve months after conversion the patient and graft survival rates were 96.7% and 95%, respectively. The incidence of biopsy-proven acute rejection was 3.3% (two cases reported, Banff grades IA and IB). The median SCr and median creatinine clearance were 168 microM (1.9 mg/dL) and 53 mL/min, respectively. Hyperlipidemia, diarrhea, peripheral edema, rash, and anemia were the most commonly reported adverse events. Patients with moderate renal insufficiency can be converted from CI to SRL-based therapy and maintain renal function over a 1-yr period.