Background: Itraconazole is often given for fungal prophylaxis to lung transplant recipients after transplantation. The aim of this study was to determine the extent of interaction between tacrolimus and itraconazole in lung transplant recipients and the efficacy of itraconazole prophylaxis. Methods: The study group included 40 lung transplant recipients followed for at least 12 months. All received prophylactic itraconazole, 200 mg twice a day, for the first 6 months after transplantation. Tacrolimus levels and dosage requirements were compared during and after itraconazole therapy. Rejection rate, fungal infection rate, and renal function were assessed. The mean cost per daily treatment of the itraconazole/tacrolimus combination and tacrolimus alone was calculated. Results: The mean tacrolimus dose during itraconazole treatment was 3.26 ± 2.1 mg/day compared with 5.74 ± 2.9 mg/day after itraconazole was stopped (p < 0.0001) for a mean total daily dose elevation of tacrolimus of 76%. When the cost of itraconazole was taken into account, the average total daily cost of the combined treatment was US$5.86 less than the treatment with tacrolimus alone. No differences in the rejection or fungal infection rate, or in renal toxicity, were observed between the periods with and without itraconazole treatment, although less positive fungal isolates were identified during itraconazole therapy. Conclusion: Prophylaxis therapy with itraconazole is highly effective. Itraconazole reduces the dose of tacrolimus and therefore lowers the cost of therapy without causing an increase in rejection rate and with renal function preservation.