BACKGROUND Bone stress injury (BSI) in youth runners is clinically important during times of skeletal growth and is not well studied. OBJECTIVE To evaluate the prevalence, anatomical distribution, and factors associated with running-related BSI in boy and girl middle school runners. DESIGN Retrospective cross-sectional study. SETTING Online survey distributed to middle school runners. METHODS Survey evaluated BSI history, age, grade, height, weight, eating behaviors, menstrual function, exercise training, and other health characteristics. MAIN OUTCOME MEASUREMENTS Prevalence and characteristics associated with history of BSI, stratified by cortical-rich (eg, tibia) and trabecular-rich (pelvis and femoral neck) locations. PARTICIPANTS 2107 runners (n = 1250 boys, n = 857 girls), age 13.2 ± 0.9 years. RESULTS One hundred five (4.7%) runners reported a history of 132 BSIs, with higher prevalence in girls than boys (6.7% vs 3.8%, p = .004). The most common location was the tibia (n = 51). Most trabecular-rich BSIs (n = 16, 94% total) were sustained by girls (pelvis: n = 6; femoral neck: n = 6; sacrum: n = 4). In girls, consuming