This study investigated the effect of calcium supplementation in preventing bone stress injuries. Healthy male military recruits (N = 1,398) served as subjects, of which 247 were randomly allocated to an experimental group (E) while 1,151 served as a control group (C). For 9 weeks both groups wore the same footwear and had the same physical training program. The baseline dietary intake of calcium in 50 randomly selected subjects of each group was assessed using a 24-hr dietary record. The E group received a daily calcium supplement while the C group did not. Injuries were monitored in all subjects by a panel of doctors who followed specific diagnostic criteria. The mean weekly injury incidence for all overuse injuries, but specifically tibial stress syndrome and stress fractures, was similar in both groups. Mean baseline daily dietary calcium intake was above 800 mg in both subgroups. This study demonstrated that large-scale calcium supplementation (500 mg/day) beyond usual dietary intake did not influence the risk of developing bone stress injuries during a 9-wk physical training program in these young military recruits.