The pathogenesis of obliterative bronchiolitis and bronchiolitis obliterans syndrome is believed to involve alloimmune and nonimmune factors. Therefore, immunosuppressive medications given either as part of the initial regimen after lung transplantation, or for recipients with recurrent or refractory acute lung rejection, might reasonably be expected to delay or prevent the later development of these complications. The use of antilymphocyte antibody therapy, preferential treatment with tacrolimus over cyclosporine A, and substitution of mycophenolate mofetil for azathioprine have all been evaluated favorably as initial immunosuppressive therapy, but definitive evidence of superiority for any of these approaches is lacking at present. For patients with recurrent or refractory acute rejection, changing cyclosporine A to tacrolimus and addition of low-dose methotrexate to the immunosuppressive regimen have been found to be of benefit. Approaches which are currently being evaluated to prevent the development of bronchiolitis obliterans include the use of rapamycin and related compounds, local administration of immunosuppressive medications, and photopheresis.