There are 300,000 to 1,215,000 high-school football injuries each year in the United States. These injuries have an important effect on player participation and health care costs. This study investigates what portion of injuries occur during the third quarter of a game, and if completing a warm-up and stretching routine after halftime reduces the incidence of third-quarter injuries. Intervention-group teams participated in a prescribed three-minute warm-up and stretching routine following the halftime break. The control group received no warm-up and stretching intervention. Fifty-five games with 108 total injuries were examined. Overall, ligament sprains and muscle strains were the most common type of injury (38%). In the nonintervention group, injuries occurred most often in the third quarter. Intervention teams sustained significantly fewer third-quarter sprains and strains per game (p less than 0.05), although no significant difference in total third-quarter injuries was noted. These findings suggest an association between post-halftime warm-up and stretching and reduced third-quarter sprain and strain injuries. We suggest a larger-scale, randomized confirmatory study.