Despite the well-recognized advantages of breast-feeding, a rare but life-threatening complication may occur with failure to thrive resulting in severe malnourishment or hypernatremic dehydration. We report 3 more cases. Two of them with severe hypernatremic dehydration and cachexia were treated using peritoneal dialysis rather than intravenous fluids in order to rapidly correct the severe metabolic acidosis but slowly the serum osmolarity. Although insufficient breast-milk volume appears to prevail over positive sodium balance in the pathway to hypernatremic dehydration, high sodium concentrations were found in breast milk as previously reported. However, the discovery of a low lactose content inversely related to that of sodium in breast-milk, suggests more complex disturbances in lactation and may draw up new hypothesis in the pathogenesis of this challenging syndrome.