Individuals with Alzheimer's disease (AD) have been shown to have abnormalities in response to fluid restriction. Twelve subjects with AD and ten elderly controls underwent overnight fluid restriction followed by measurement of plasma and urine vasopressin and serum osmolality. Estimates of “thirst” were determined after one hour of ad libitum water intake. All subjects were tested with a Mini-Mental State Examination (MMSE) and Global Deterioration Scale (GDS). Individuals with AD had a greater degree of overnight dehydration than the elderly control group (serum osmolality 310 +/−1 vs. 305 +/−1 mosmol/kg, p = 0.02). There was no difference between the groups in the plasma or urinary levels of vasopressin. There was a direct correlation (r = 0.45, p = 0.03) of the amount of water intake as a measure of “thirst” with the MMSE score as a measure of cognitive functioning. Individuals with advanced cognitive impairment may be at risk of dehydration due to loss of protective “thirst” responses with secondary complications of dehydration.