STUDY DESIGN: Prospective cohort. OBJECTIVES: To examine the relationship between summer training practices and risk of injury during the first month of a high school interscholastic cross-country season. BACKGROUND: Several prospective studies have reported a high incidence of injury in adolescent cross-country runners. However, limited reports exist on the role of summer training practices and risk of injury among these runners. METHODS: Four hundred twenty-one athletes (186 girls, 235 boys) who competed in interscholastic cross-country were followed during a cross-country season. At the start of the season, all participants completed a questionnaire regarding summer training routines. Time-loss, running-related injuries were tracked during the subsequent season. Logistic regression analysis was used to estimate the odds ratios (ORs) and 95% confidence intervals (CIs) for the risk of initial injury during the first month of the season associated with summer training variables. RESULTS: Sixty-seven runners (15.9%) had a confirmed injury during the first month of the season, with a higher percent among girls (19.4%) than boys (13.2%) (P = .06). Overall, 60.1% of the participants ran during the summer prior to the season, with a significantly higher percent among girls (71.5%) than boys (51.1%) (P<.0001). Overall, no significant association (OR = 0.9; 95% CI:0.5, 1.5; P = .90) was found between not running sometime during the preceding summer and increased risk of initial injury during the first month of the season. Among only the runners who ran during the summer, after adjusting for sex and prior injury, first-month injuries were more common among those who did not frequently alternate short and long mileage on different days (OR = 3.0; 95% CI: 1.4, 6.4; P = .005), and/or who ran 8 weeks or fewer (OR = 2.7; 95% CI:1.2, 5,8; P = .01) during their summer training. Running 8 weeks or fewer (P = .03), not frequently alternating mileage on different days (P = .01), and running a higher percentage of time on predominantly hill (P = .001) and irregular terrains (P = .004) were associated with increased risk of injury for girls. CONCLUSION: These findings suggest that injuries during the first month of the high school cross-country season may be reduced if runners who participate in summer training activities run a greater number of weeks and frequently vary their daily running mileage during the summer. For girls, training programs that reduce mileage on hills and irregular terrains may help to minimize the occurrence of running-related injury.