It is well accepted that hyperventilation before breath hold swimming and diving makes it possible for a person to extend the time under water. Less well known is the fact that this maneuver can cause loss of consciousness due to hypoxia. This accident happens almost exclusively to males (56 cases). The most common age group was 16-20 years (range 12-33 years). All were known to be good swimmers or divers. Approximately 80% of the cases occurred in guarded pools. Thirty-five subjects survived the accident and of the twenty-three fatalities, there was only one good autopsy report. In this instance the findings were those associated with classical drowning preceded by hypoxia and hypercapnia. Breath holding experiments indicated that the times between loss of consciousness and death may be no longer than 2.5 minutes. The patterns associated with these cases suggest that those who are responsible for aquatic safety as supervisors or guards of pools could prevent most accidents by watching for young male swimmers who are practicing hyperventilation and underwater swimming in competition with themselves or with others.