Both acute and chronic fluid deficits have been shown to be associated with a number of adverse health outcomes. At the extreme, deprivation of water for more than a few days inevitably leads to death, but even modest fluid deficits may precipitate adverse events, especially in young children, in the frail elderly and in those with poor health. Epidemiological studies have shown an association, although not necessarily a causal one, between a low habitual fluid intake and some chronic diseases, including urolithiasis, constipation, asthma, cardiovascular disease, diabetic hyperglycemia, and some cancers. Acute hypohydration may be a precipitating factor in a number of acute medical conditions in elderly persons. Increased mortality, especially in vulnerable populations, is commonly observed during periods of abnormally warm weather, with at least part of this effect due to failure to increase water intake, and this may have some important implications for those responsible for forward planning in healthcare facilities.