We report a case of overdrinking-induced hyponatraemia from the 2007 London Marathon. The patient was a 37-year-old experienced female marathon runner. She was brought to the emergency room more than 6 h after completing the marathon suffering from diarrhoea, vomiting and confusion, and was unable to recall any detail of the race. An arterial blood sample confirmed hyponatraemia ([Na(+)] 117 mmol.l(-1)) associated with hypokalaemia (serum potassium concentration 3.4 mmol.l(-1)) and respiratory alkalosis (pH 7.62, bicarbonate 16.1 mmol.l(-1) and Pco(2) 2.14 kPa). A diagnosis of uncomplicated exercise-associated hyponatraemia due to voluntary overdrinking was made and the patient was catherised and treated with a slow (1 h) intra-venous infusion of 500 ml of 1.8% sodium chloride (NaCl) solution. The following morning her serum [Na(+)] had normalised at 135 mmol.l(-1) and she was discharged in the afternoon. She has recovered fully without sequelae.