Individuals with dyslexia have difficulties in processing speech sounds (termed phonemes). This functional MRI study showed that the left medial geniculate body (MGB), but no other components of the auditory pathway, responds aberrantly when adults with dyslexia were asked to attend to phonemes compared with other speech features. Moreover, MGB activity correlated with dyslexia severity. This finding may help to clarify the underlying causes of dyslexia.