Ketoconazole inhibits cytochrome P 3A4, leading to a 10-fold increase in sirolimus blood levels. Although it has not been reported in the clinical setting so far, sirolimus and ketoconazole co-prescription can lead to cost saving by reducing the dose of sirolimus administered. After informed consent was obtained, sirolimus and ketoconazole co-prescription was studied in six patients who could not afford the current recommended doses. Patients received one-eighth to one-fourth of the recommended dose of sirolimus (0.25-0.5 mg) with 100 to 200 mg of ketoconazole. Sirolimus levels were monitored, and the dose of ketoconazole was increased to achieve target levels of sirolimus. The loading dose was 3 mg of sirolimus with 100 mg of ketoconazole. After sirolimus rescue therapy was started, serum creatinine decreased in five patients. The mean serum creatinine for the group decreased from 2.6 +/- 0.3 mg/dL at the initiation of rescue therapy to 2.2 +/- 0.5 mg/dL on the last follow-up. Sirolimus ketoconazole co-prescription with monitoring of sirolimus levels is possible and safe and needs to be explored further.