BACKGROUND: The identification of fluctuating confusion is central to improving the differential diagnosis of the common dementias. AIMS: To determine the value of two rating scales to measure fluctuating confusion. METHOD: The agreement between the clinician-rated scale and the scale completed by a non-clinician was determined. Correlations between the two scales were calculated; variability in attention was calculated on a computerised cognitive assessment and variability in delta rhythm on an electroencephalogram (EEG). RESULTS: The Clinician Assessment of Fluctuation and the computerised cognitive assessment were completed for 155 patients (61 Alzheimer's disease, 37 dementia with Lewy bodies, 22 vascular dementia, 35 elderly controls). A subgroup (n = 40) received a further evaluation using the One Day Fluctuation Assessment Scale and an EEG. The two scales correlated significantly with each other, and with the neuropsychological and electrophysiological measures of fluctuation. CONCLUSIONS: Both scales are useful instruments for the clinical assessment of fluctuation in dementia.