FK506 is being used increasingly to prevent rejection after organ transplantation. Its use is associated with a wide spectrum of neurotoxicity, which has been described after most solid organ transplantations, but reports after lung transplantation are extremely rare. This is a report of the pathologic correlation of the clinical and radiologic features of delayed FK506-induced fulminant leukoencephalopathy after single-lung transplantation. The patient presented with neurologic symptoms that progressed to seizure activity. Neuroimaging showed diffuse changes in the brain, and results of a brain biopsy were consistent with leukoencephalopathy with microglial and astrocytic activation. The patient had a remarkable improvement in clinical status after discontinuation of FK506 administration, with resolution of the changes seen on neuroimaging.