The P50 (or P1) potential is a midlatency auditory evoked response, believed to be partially generated by the cholinergic pedunculopontine nucleus (PPN) neurons that give rise to the ascending reticular activating system (RAS). We compared the P50 potential in 11 Huntington's disease (HD) patients and 13 normal controls using a paired click stimulus paradigm. HD patients exhibited a P50 potential with reduced amplitude and increased latency to the first stimulus of the pair (first P50 potential), suggesting impaired ascending reticular activating system function, which may contribute to sleep disorders seen in HD. Sensory gating, measured as the percent ratio of the second P50 potential amplitude to the first P50 potential amplitude, was reduced at 250 and 500 ms interstimulus intervals (ISI), which may be related to disordered attention and anxiety in HD.