Background: Most breast-fed newborns get the milk they need. However, very rarely milk intake is insufficient mostly as a result of poor breastfeeding techniques. Dramatic weight loss and hypernatremic dehydration may occur. Our aim was to construct charts for weight loss. Methods: A case-control study was performed. Charts with standard deviation score (SDS) lines for weight loss in the first month were constructed for 2,359 healthy breast-fed term newborns and 271 cases with breastfeeding-associated hypernatremic dehydration with serum sodium level > 149 mEq/L. Day 0 was defined as the day of birth. Results: Many cases with (or who will develop) hypernatremic dehydration (84%; +1 SDS line) fell below the -1 SDS line at day 3, the -2 SDS line at day 4, and the -2.5 SDS line at day 5 in the chart of the healthy breast-fed newborns. Weight loss of cases with permanent residual symptoms was far below the -2.5 SDS. Conclusions: Already at an early age, weight loss differs between healthy breast-fed newborns and those with hypernatremic dehydration. Charts for weight loss are, therefore, useful tools to detect early, or prevent newborns from developing, breastfeeding-associated hypernatremic dehydration, and also to prevent unnecessary formula supplementing. © 2014 Wiley Periodicals, Inc.