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To identify risk factors for stress fracture among young female distance runners.Participants were 127 competitive female distance runners, aged 18-26, who provided at least some follow-up data in a randomized trial among 150 runners of the effects of oral contraceptives on bone health. After completing a baseline questionnaire and undergoing bone densitometry, they were followed an average of 1.85 yr.Eighteen participants had at least one stress fracture during follow-up. Baseline characteristics associated (P < 0.10) in multivariate analysis with stress fracture occurrence were one or more previous stress fractures (rate ratio [RR] [95% confidence interval] = 6.42 (1.80-22.87), lower whole-body bone mineral content (RR = 2.70 [1.26-5.88] per 1-SD [293.2 g] decrease), younger chronologic age (RR = 1.42 [1.05-1.92] per 1-yr decrease), lower dietary calcium intake (RR = 1.11 [0.98-1.25] per 100-mg decrease), and younger age at menarche (RR = 1.92 [1.15-3.23] per 1-yr decrease). Although not statistically significant, a history of irregular menstrual periods was also associated with increased risk (RR = 3.41 [0.69-16.91]). Training-related factors did not affect risk.The results of this and other studies indicate that risk factors for stress fracture among young female runners include previous stress fractures, lower bone mass, and, although not statistically significant in this study, menstrual irregularity. More study is needed of the associations between stress fracture and age, calcium intake, and age at menarche. Given the importance of stress fractures to runners, identifying preventive measures is of high priority.