Objective:To draw attention to the occurrence of hypophosphataemia in anorexia nervosa and to suggest that it may be a factor contributing to cardiac arrhythmia, a potentially fatal complication.Design:A retrospective study of patients under the consultant care of one of us. All were adolescent girls or young women aged 14 to 31.Clinical course:During the index admission, three became acutely hypophosphataemic while receiving saline and potassium intravenously to correct hypokalaemia. Four patients were subsequently treated with phosphate supplementation. Two patients died, and three recovered.Conclusion:Hypophosphataemia in anorexia nervosa patients may play a role in the development of cardiac arrhythmia and delirium.