Objective.—To document the prevalence of hypothermia in a mass participation endurance open water swimming event and to determine demographic and individual factors that may predict failure to finish the race and hypothermia.

Methods.—A prospective observational study in competitors in a 19.2-km open water swimming race in Perth, Western Australia. Pre-race information collected included age, sex, training and race experience, medical history, and body mass index (BMI). Body temperatures at 5 minutes postrace were measured using an equilibrated oral- or rectal-reading low-range glass mercury thermometer. Logistic regression was used to develop models predicting hypothermia (defined as a temperature of <35°C) and failure to finish the race.

Results.—One hundred and nine competitors (70 male, 39 female) with a combined mean age of 38.4 ± 12.1 years were studied. Hypothermia was the most common race-related illness, identified in 26 of 35 swimmers screened as requiring temperature measurement, including 5 who required short-stay hospital care and 2 who required critical care transfer. Longer race duration (odds ratio [OR] 1.77, 95% CI 1.10–2.84, P = .018) was associated with an increased risk of hypothermia, and higher BMI (OR 0.57, 95% CI 0.41–0.79, P = .001) was associated with a decreased risk of hypothermia. Weak predictors of failure to finish were age (OR 1.06, 95% CI 1.01–1.11, P = .012) and hours spent training (OR 1.08, 95% CI 1.01–1.16, P = .025).

Conclusions.—Hypothermia is a common condition affecting mass participation long-distance open water swimmers. Increased BMI appears to be protective against hypothermia, while prolonged duration of the swim predicts an increased risk of hypothermia. The weak predictors of failing to finish are of questionable clinical significance.