We reported a 15-year-old boy with an acute myelomonocytic leukemia and FK 506-induced leukoencephalopathy. He was received FK 506 for graft versus host disease occurred after peripheral blood stem cell transplantation. He, four weeks later, had generalized seizures and consciousness disturbance. The serum level of FK 506 was high (27.5 ng/ml). His brain MRI showed abnormal high intensity areas in the frontal and parietal white matter lesions on T2-weighted images. Neuropathological studies revealed the destruction of myelin sheeths and axons in the cerebral white matter corresponded with abnormal lesions on MRI. There were calcification and mineralization in the small vessel walls of the cortex and white matter. Osteopontin immunoreactivity was detected in the endothelial cells of small vessels. These findings suggest that the vascular damage was involved in the FK 506-induced leukoencephalopathy.