ObjectiveTo evaluate a method of medical care at an ultradistance triathlon, with the aim of reducing the incidence of hyponatremia.DesignDescriptive research.SettingNew Zealand Ironman triathlon (3.8 km swim, 180 km cycle, 42.2 km run).Participants117 of 134 athletes seeking medical care after the triathlon (involving 650 race starters).InterventionsA prerace education program on appropriate fluid intake was undertaken. The number of support stations was decreased to reduce the availability of fluid. A body weight measurement before the race was introduced as a compulsory requirement, so that weight change during the race could be included in the triage assessment. An on-site laboratory was established within the race medical tent.Main Outcome MeasuresNumbers of athletes and diagnoses, including the incidence of symptomatic hyponatremia (defined as symptoms of hyponatremia in association with a pretreatment plasma sodium concentration [Na] < 135 mmol/L); weight changes; and changes in [Na].ResultsThe common diagnoses in the 117 athletes receiving attention were exercise-associated collapse (27%), musculoskeletal complaints (26%), and dehydration (12%). There was a significant reduction in the number of athletes receiving medical care for hyponatremia, from 25 of the 114 athletes who received care in 1997 (3.8% of race starters) to 4 of the 117 athletes who received care in 1998 (0.6% of race starters). Mean weight change among athletes in the 1998 race was −3.1 kg, compared with −2.6 kg in 1997.ConclusionA preventive strategy to decrease the incidence of hyponatremia, including education on fluid intake and appropriate placement of support stations, was associated with a decrease in the incidence of symptomatic hyponatremia.