Realtime two-dimensional ultrasonography was used to observe, through the anterior fontanelle, the brain of a patient with Leigh's encephalomyelopathy. The patient, an 11-month old boy, was normal during his first 6 months. Subsequently, however, his development slowed drastically, and he lost his head control. Examination revealed metabolic acidosis with elevated levels of lactate and pyruvate, and the ultrasonogram showed bilateral hyperechoic lesions of the putamen, thalamus, caudate nucleus, corpus callosum, and medulla. CT scan showed these lesions to be low density areas. Neuropathological abnormalities of Leigh's encephalomyelopathy are known to be vascular dilatation and capillary proliferation with spongy changes of ground substance. With ultrasonography the lesions are disclosed as hyperechoic areas like normal choroid plexus. Histologically, choroid plexus appears as a leaflike projection (villi) with tortuous capillaries and loose connective tissue. Echogenicity in these examinations may have been produced by differences in acoustic impedance between partition structures such as capillary or epithelial walls, and fluid components such as blood, interstitial fluid, and CSF. It is suggested that the close arrangement of capillary walls may produce the hyperechoic lesions of Leigh's encephalomyelopathy.