Objectives To review the presentation, treatment and response of those runners from the London Marathon who presented to St Thomas’ Hospital with exercise induced hyponatraemia. Design Observational case series. Setting St Thomas’ Hospital, a tertiary hospital situated near the finish line of the 2003 London Marathon. Participants All runners who presented to St Thomas’ Hospital on the day of the 2003 London Marathon with altered mental state whose serum sodium concentration was less than 135 mmol/L. Main outcome measures Presenting symptoms, volume and type of fluids administered and response to treatment (biochemical and clinical). Results Fourteen patients were diagnosed with exercise associated hyponatraemia with serum sodium concentrations ranging from 116 to 133 mmol/L. Eleven presented with confusion. There were long delays between the finish time and presentation time for some runners. Anecdotal descriptions suggested some runners finished the race with normal mental state then became confused. There was no correlation between running time and serum sodium level. All patients received 0.9% saline and six received 1.8% saline. Despite this, some patients demonstrated falls in serum sodium concentrations. Thirteen to fourteen patients were symptomatically well the following morning, with the remaining patient significantly improved. Conclusion Presentation of exercise associated hyponatraemia may be delayed. Optimal treatment is controversial, but the use of isotonic saline may not result in rises of serum sodium and we would suggest the early use of hypertonic fluids in symptomatic patients.