The role played by hydration in general health and well-being is an emerging public health issue, yet there are few tools available to monitor its status in large populations. The aim was therefore to develop a questionnaire that assesses individual differences in the tendency to lose body fluid in a warm environment and hence become dehydrated. Fifty-three subjects sat in a room at 30°C for four hours and changes in mood and measures of hydration were monitored. There were marked individual differences in the loss of body mass that differed from 0.24% to 2.39%. Females who reported habitually drinking a lot had more water in their diet and at baseline the osmolality of urine was lower. After being subject to heat, those who reported habitually drinking more produced more urine, had a lower urine osmolality at the end of the study, and overall more body mass was lost. Females who reported that they responded badly to heat were more confused, unsure and depressed after four hours at 30°C. In males those reporting that they habitually drank to a greater extent had more water in the diet, and also those who dealt badly with heat habitually drank more. It was concluded that particularly in females, questionnaire measures were able to predict changes in hydration that result from a warm environment.