Bronchiolitis obliterans syndrome (BOS) continues to be the main factor limiting the long-term survival of lung transplant recipients. The objective of this study was to prospectively assess the impact of conversion from cyclosporine (CsA) to tacrolimus on lung function in patients who developed BOS while receiving CsA-based immunosuppressive therapy. A total of 79 patients with BOS were included in the study. Sixty percent of patients had stage II or III BOS according to the International Society for Heart and Lung Transplantation criteria. Mean time from transplantation was 30.4 +/- 21.9 months and all patients were on CsA therapy at enrollment in the study, with mean trough levels of 232.75 +/- 98.26 ng/mL. After conversion, tacrolimus trough levels were 11.0 +/- 3.6 ng/mL at 3 months and 9.0 +/- 3.4 ng/mL at 12 months. Sixteen deaths occurred during the first year postconversion, 56% of which were due to respiratory failure. Comparison of forced expiratory volume in 1 second (FEV(1)) preconversion versus postconversion showed a change in the slope of the FEV(1)-time curve. The slope of the preconversion curve was -0.44 versus a zero slope, whereas the slope of the postconversion curve was 0.005, with a statistically significant difference between both slopes. This change in slopes, which was also seen in FEV(1%), suggests that lung function loss closed after conversion from CsA to tacrolimus supporting this therapeutic strategy in lung transplant recipients with BOS treated with CsA.