The pathophysiology of adult sleepwalking is still poorly understood. However, it is widely accepted that sleepwalking is a disorder of arousal. Arousal circuits widely project to the cortex, including motor cortex. We hypothesized that functional abnormality of these circuits could lead to changes in cortical excitability in sleepwalkers, even during wakefulness. We used transcranial magnetic stimulation (TMS) to examine the excitability of the human motor cortex during wakefulness in a group of adult sleepwalkers. When compared with the healthy control group, short interval intracortical inhibition (SICI), cortical silent period (CSP) duration, and short latency afferent inhibition (SAI) were reduced in adult sleepwalkers during wakefulness. Mean CSP duration was shorter in patients than in controls (80.9 +/- 41 ms vs. 139.4 +/- 37 ms; p = 0.0040). Mean SICI was significantly reduced in patients than in controls (73.5 +/- 38.4% vs. 36.7 +/- 13.1%; p = 0.0061). Mean SAI was also significantly reduced in patients than in controls (65.8 +/- 14.2% vs. 42.8 +/- 16.9%; p = 0.0053). This neurophysiological study suggests that there are alterations in sleepwalkers consistent with an impaired efficiency of inhibitory circuits during wakefulness. This inhibitory impairment could represent the neurophysiological correlate of brain "abnormalities" of sleepwalkers like "immaturity" of some neural circuits, synapses, or receptors.