Five cases of moderately severe hypernatraemic dehydration were identified within a 5-month period between two regional hospitals in Hong Kong. Unlike previous reported cases, these exclusively breast-fed infants presented with the unusual triad of fever, absence of overt signs of dehydration and within the first week of life. Three of the cases also had high serum bilirubin concentrations at presentation. The fever subsided quickly and the serum bilirubin concentration fell rapidly within a few hours of rehydration. All infants made an uneventful recovery without permanent neurological sequelae. Fever, presumably secondary to dehydration, is an useful early warning sign. These cases emphasize the importance of early and regular measurement of bodyweight in exclusively breast-fed infants so that prompt identification of affected cases may prevent potentially detrimental complications.