Background: It has been hypothesized that a period of rest from running in the early weeks of basic military training will prevent stress fractures among recruits. Design: Modification of running schedules in companies of Army recruits undergoing basic military training was assigned. Setting/ Participants: Six male training companies were enrolled and followed during their 8 weeks of basic military training at Fort Bliss, Texas, in summer/fall 1989. Intervention: Intervention companies were asked to rest from running during the second, third, or fourth week of basic military training. Main outcome measures: Data were collected from questionnaires, anthropometric measurements, Army physical fitness tests, company training logs, and medical record abstraction of all clinic visits. Results: Among the 1357 enrolled male recruits, there were 236 (17%) with overuse injury and 144 (11%) with traumatic injury, resulting in 535 clinic visits and 1927 training days lost. Stress fracture/reaction rates varied from 3 to 8 per 100 recruits among the intervention companies and 2 to 7 per 100 recruits among the non-intervention companies. Total injury rates were 18 to 35 per 100 recruits in the intervention companies and 18 to 29 per 100 recruits in the non-intervention companies. Conclusions: The study provided no evidence for a protective effect on overuse injuries of resting from running for 1 week early in basic military training. There was varied physical training among the companies, however, with variation of injury rates that likely related to factors other than the intervention.