Motor evoked potentials (MEPs) to focal transcranial magnetic stimulation (TMS) have demonstrated that abnormal ipsilateral corticospinal projections are active in patients with congenital mirror movements. In addition, movement-related potentials and PET suggest that an abnormal pattern of motor cortex activation could be associated with an anomaly of the corticospinal tracts. In the present study the silent period (SP) following focal TMS was investigated in a woman with familial congenital mirror movements. Recordings were made from both abductor pollicis brevis (APB) muscles. When focal TMS was delivered during an intended contralateral APB muscle contraction, MEP and SP were bilaterally recorded and SP was significantly shorter than the contralateral SP observed in normal controls. An abnormal bilateral activation of the hand motor cortex can explain our findings. The non-stimulated motor cortex causes an early partial recovery of the background EMG activity when the stimulated motor cortex is still inhibited (beginning as soon as the transcallosal and the short-lasting segmental inhibition are both complete).