To our knowledge, we are the first to report the diffusion-weighted MR imaging findings in a 15-day-old neonate with nonketotic hyperglycinemia. We found bilaterally symmetrical lesions of restricted diffusion in the dorsal brain stem, cerebral peduncles, and posterior limbs of the internal capsule, which were more conspicuous and extensive on diffusion-weighted MR images than on T2-weighted images. These lesions are in the myelinated tracts of the neonate and are compatible with the expected sites of abnormality in vacuolating myelinopathy.