INTRODUCTION: We investigated two athletes swimming in 4 degrees C for 23 min (1.3 km, swimmer 1) and 42 min (2.2 km, swimmer 2), respectively. MATERIALS AND METHODS: Pre swim, percent body fat was determined; post swim, core temperature was measured. RESULTS: The core temperature of swimmer 2 was: 37.0 degrees C immediately before the start, 32 degrees C 20 min after getting out of the water, and 35.5 degrees C 80 min after finishing the swim. CONCLUSION: We assume that the higher skin-fold thickness and body fat of swimmer 2 enabled him to perform longer. In addition to this, mental power and experience in cold water swimming must be considered. In any athlete aiming at swimming in water of less than 5 degrees C, body core temperature and heart rate should be continuously monitored in order to detect a body core temperature below 32 degrees C and arrhythmia to pull the athlete out of the water before life-threatening circumstances occur.