102 children with acute gastroenteritis were thought by the admitting junior doctors to be 5% or more dehydrated. As judged by subsequent weight recovery in hospital, the main indicators of mild to moderate dehydration were decreased peripheral perfusion, deep breathing, decreased skin turgor, high urea, low pH, and a large base deficit; a history of increased thirst was just short of statistical significance. Dehydration was not indicated by a history of oliguria, by the presence of restlessness or lethargy, sunken eyes, dry mouth, or a sunken fontanelle or by the absence of tears. Clinical signs of dehydration became apparent at 3-4% rather than 5% dehydration. The degree of dehydration was overestimated by a mean of 3.2%; this caused unnecessary hospital admissions and overtreatment with intravenous fluid.