Background: Calcineurin inhibitor (CNI) immunosuppressants are a major cause of renal dysfunction in cardiac transplant recipients, leading to increased morbidity and mortality. The aim of this study was to evaluate the efficacy and safety of CNI withdrawal and substitution with sirolimus as the primary immunosuppressant, and assess the effect on renal function in cardiac transplant recipients with CNI-induced renal impairment. Methods: Thirty-four stable cardiac transplant recipients (range 1 to 14 years post-transplant) with CNI-induced nephrotoxicity (iothalamate clearance 25 to 50 ml/min) or cardiac allograft vasculopathy (CAV) were enrolled. Twelve patients (Group A) were prospectively enrolled for renal dysfunction. The remaining patients (n = 22, Group B) were converted to sirolimus on clinical grounds because of poor renal function or the presence of CAV. CNI was withdrawn gradually over 12 weeks. Sirolimus was started at 1 mg/day with titration over 2 weeks to achieve levels of 10 to 15 ng/ml. Echocardiograms and cardiac biopsies were performed to determine rejection. Adjunct immunosuppression was left unchanged. Follow-up iothalamate clearance was performed. A further 24 patients (Group C) were retrospective controls, stable (range 2 to 10 years post-transplant), and maintained on a standard CNI-based immunosuppressant regimen. Results: Iothalamate clearance (Ci) improved significantly (Group A baseline: 36.08 ± 2.4 ml/min to 48.67 ± 4.1 ml/min, p = 0.004; Group B baseline: 48.14 ± 3.2 ml/min to 55.77 ± 4.2 ml/min, p < 0.001) without exacerbating rejection or compromising cardiac function. By contrast, in controls, Group C, the baseline renal clearance declined from 40.04 ± 1.86 ml/min to 34.63 ± 1.6 ml/min over the course of 1 year (p < 0.01). Conclusions: Substitution of CNIs with sirolimus in cardiac transplant recipients is safe and effective and leads to an improvement in renal function, without compromise in cardiac function and rejection.