Two children with hemimegalencephaly were examined by magnetic resonance imaging (MRI) and localized proton MR spectroscopy (MRS). In both cases, structural changes in the enlarged hemisphere included pachy- or polymicrogyria and gliosis of white matter. Associated metabolic disturbances included a dramatic reduction of glutamate and N-acetylaspartate (NAA) in white matter. Less severe or no alterations were noted in cortical gray matter, basal ganglia, and cerebellum. The older child (13 years) showed increased myoinositol in both gray and white matter as well as markedly increased choline-containing compounds in gray matter. Both children also had mildly decreased NAA levels in the white matter of the contralateral hemisphere. The spectroscopic findings indicate loss of vital neuroaxonal tissue and glial cell proliferation. Metabolic disturbances were more pronounced in the older child. The normal-appearing hemisphere was mildly affected in both cases.