Hyperventilation by swimmers can lead to hypoxia and loss of consciousness. In this retrospective case series we describe the simultaneous onset of hypoxia in multiple 13-15-year-old female synchronized swimmers of an elite synchronized swim team based on review of Health Department and emergency medical service reports. All six swimmers performing hypoxic drills developed hypoxic symptoms (fatigue, inability to move legs, disorientation, tunnel vision, and/or loss of consciousness) and four required rescue. All swimmers regained consciousness at the scene and recovered. Two swimmers not performing hypoxic drills were unaffected. Of the environmental evaluations, only pool water temperature was higher than the recommended levels. Hyperventilation with hypoxic training can be life-threatening and should be prohibited. Duration of hypoxic drills should be limited. Emergency response plans should be practiced.