An electromagnetic coil applied to the head that emits pulses of skull-penetrating, neuron-activating magnetic energy generates transcranial magnetic stimulation. De Beaumont et al13used transcranial magnetic stimulation with motor-evoked responses detected by an electromyogram recorder attached to an index finger muscle. With different protocols, they measured resting motor threshold, intracortical inhibition and facilitation, excitability of the corticospinal system, and the duration of a cortical silent period. Control, singly concussed, and multiply concussed groups were studied, with all concussion events having occurred more than 9 months before study. Abnormality of the intracortical inhibitory system, manifested by increased cortical silent period response, was seen in the multiply concussed athletes (who had also sustained the most severe concussions). The best correlation for increased silent period response proved to be the severity of prior concussion. These results show that multiple sports concussions may result in functional brain abnormality not detected by symptom scores or neuropsychological measurements persisting for more than 9 months.