Abnormal auditory processing in dyslexics suggests that accompanying anatomical abnormalities might be present in the auditory system. Therefore, we measured cross-sectional neuronal areas in the medial geniculate nuclei (MGNs) of five dyslexic and seven control brains. In contrast to controls, which showed no asymmetry, the left-side MGN neurons were significantly smaller than the right in the dyslexic sample. Also, as compared with controls, there were more small neurons and fewer large neurons in the left dyslexic MGN. These findings are consistent with reported behavioral findings of a left hemisphere-based phonological defect in dyslexic individuals.