Fluctuating levels of consciousness (FC) are a key feature in neurodegenerative dementias, yet clinical identification is poor, hindering accurate diagnosis. One hundred and nineteen patients (32 Dementia with Lewy Bodies (DLB), 57 Alzheimer's disease (AD) and 30 controls) with clinical scores of FC were assessed using an attentional task. Cortical arousal was assessed in 25 of these patients using electroencephalography. Over 90 s both variability in attention (p<0·0001) and fluctuations in electrocortical activity (p<0·0001) correlated with clinical FC scores, and with each other (p<0·0001). Variability in attention and electrocortical arousal are accurate FC markers and can assist differential diagnosis of AD and DLB. Previous work has underestimated the intensity and hence impact of FC in dementia. Copyright © 1999 John Wiley & Sons, Ltd.