Context: A recent study reported that drinking 500 ml water causes a 30% increase in metabolic rate. If verified, this previously unrecognized thermogenic property of water would have important implications for weight-loss programs. However, the concept of a thermogenic effect of water is controversial because other studies have found that water-drinking does not increase energy expenditure. Objective: To test whether water-drinking has a thermogenic effect in humans and, furthermore, to determine whether the response is influenced by osmolality or by water temperature. Design: A randomized cross-over design. Setting: University physiology laboratory. Participants: Healthy young volunteer subjects. Intervention: Drinking 7.5 ml/kg body weight (518 ml) of distilled water, 0.9% saline or a 7% sucrose solution (positive control), on different days. In a sub-group of subjects, responses to cold water (3 C) were tested. Main Outcome Measure: Resting energy expenditure, assessed by indirect calorimetry for 30 min before and 90 min after the drinks. Results: Energy expenditure did not increase after drinking either distilled water (P = 0.34) or 0.9% saline (P = 0.33). Drinking the 7% sucrose solution significantly increased energy expenditure (P < 0.0001). Drinking water that had been cooled to 3 C caused a small increase in energy expenditure of 4.5% over 60 min (P < 0.01). Conclusion: Drinking distilled water at room-temperature did not increase energy expenditure. Cooling the water before drinking only stimulated a small thermogenic response, well below the theoretical energy cost of warming the water to body temperature. These results cast doubt on water as a thermogenic agent for the management of obesity.