Among a population of randomly selected US Marine Corps recruits at Marine Corps Recruit Depot, San Diego, California, the authors developed a screening tool to identify individuals at high risk for lower extremity stress fracture when beginning a rigorous physical training program. The screening tool was developed among 1,286 recruits, then tested and refined among 1,078 additional recruits. The refined algorithm, consisting of five physical activity questions and a 1.5-mi (2.4-km) run time, revealed that 21.6% of "high risk" subjects suffered more than three times as many stress fractures as "low risk" subjects. These data suggest that risk of stress fracture during rigorous physical training is increased by poor physical fitness and low levels of physical activity prior to their entry into the program.