We report a series of 26 heart transplant recipients with renal impairment in which sirolimus was used as the basic immunosuppresive drug (without associated calcineurin inhibitors) to avoid further nephrotoxicity. Sirolimus (trough levels 10 to 12 ng/mL, average daily dose 3 mg) was used in two settings: de novo in 7 patients with significant preexistent renal impairment and as a chronic conversion in 19 stable patients with established renal failure (creatinine level >2 mg/dL). In all de novo patients (n = 7), the renal function significantly improved. Creatinine fell from 2.95 +/- 0.9 mg/dL to 1.41 +/- 0.4 mg/dL at follow-up (P = .0017). One patient died suddenly of a massive pulmonary embolism. Only one patient experienced histologic but reversible rejection. In one patient, anemia and diarrhea prompted sirolimus withdrawal. Five patients had infectious episodes: three bacterial pneumonias, one mediastinitis, and two CMV infections. In the chronic conversion group (n = 19), the improvement was mostly limited to patients with moderate renal failure (creatinine < or =2.5 mg/dL) in which creatinine fell from 2.24 +/- 0.2 to 1.9 +/- 0.27 mg/dL, P = .009). When basal creatinine was over 2.5 mg/dL, only one third of the patients improved after conversion. Two patients died: terminal renal failure and cerebrovascular accident. There were no clinical episodes of rejection. Secondary effects prompted the discontinuation of sirolimus in five patients: two definite and one possible interstitial pneumonitis and two cases of anemia). The symptoms resolved after sirolimus withdrawal. Six patients had infection: four pneumonias, one sepsis, and one cutaneous abscess. Sirolimus is an interesting alternative to calcineurin inhibitors in selected patients with renal impairment. It prevents renal failure in de novo recipients at high risk of catastrophic renal damage and ameliorates renal dysfunction in chronic patients with moderate renal dysfunction. Given the high incidence of secondary effects, the adequate dosage and the secondary effects profile needs further study.