Huntington's disease (HD) is a devastating neurodegenerative disorder with prominent motor and cognitive decline. Previous studies with small sample sizes and methodological limitations have described abnormal electroencephalograms (EEG) in this cohort. The aim of the present study was to investigate objectively and quantitatively the neurophysiological basis of the disease in HD patients as compared to normal controls, utilizing EEG mapping. In 55 HD patients and 55 healthy controls, a 3-min vigilance-controlled EEG (V-EEG) was recorded during midmorning hours. Evaluation of 36 EEG variables was carried out by spectral analysis and visualized by EEG mapping techniques. To elucidate drug interference, the analysis was performed for the total group, unmedicated patients only and between treated and untreated patients. Statistical overall analysis by the omnibus significance test demonstrated significant (p < 0.01 and p < 0.05) EEG differences between HD patients and controls. Subsequent univariate analysis revealed a general decrease in total power and absolute alpha and beta power, an increase in delta/theta power, and a slowing of the centroids of delta/theta, beta and total power. The slowing of the EEG in HD reflects a disturbed brain function in the sense of a vigilance decrement, electrophysiologically characterized by inhibited cortical areas (increased delta/theta power) and a lack of normal routine and excitatory activity (decreased alpha and beta power). The results are similar to those found in other dementing disorders. Medication did not affect the overall interpretation of the quantitative EEG analysis, but certain differences might be due to drug interaction, predominantly with antipsychotics. Spearman rank correlations revealed significant correlations between EEG mapping and cognitive and motor impairment in HD patients.