The elderly are at risk of developing disturbances of water homeostasis, and clinicians have to rely on laboratory measurement to determine their presence and magnitude. Optimum management depends on having a clear understanding of age-associated changes in water homeostasis. Many studies have flawed methodology, but some conclusions can be drawn. There is a diminished thirst, but the relationship of plasma osmolality to arginine vasopressin is at least preserved, and may show increased responsiveness. There is reduced renal function with age and the kidneys' ability to produce a concentrated urine declines. Similarly, the excretion of a water load becomes impaired. Further study is needed of the interaction of specific diseases with these age-associated changes.