Base deficit is a parameter often used to guide further treatment in acidotic children and is taken as a measure of how "sick" they are. Five children with septic shock are presented who had persisting base deficit after large volume resuscitation with 0.9% saline. Stewart's strong ion theory of acid-base balance is able to quantify the causes of metabolic acidosis and is used to show that our patients had a hyperchloraemic metabolic acidosis. We show how the chloride content of the saline loads given to our patients caused this hyperchloraemia. It is concluded that 0.9% saline and other chloride rich fluids may not be ideal resuscitation fluids; if used, clinicians must be aware of their potential to cause a persistent base deficit.