OBJECTIVE Previous literature suggests that injury rates during physical activity may be higher in the summer than in the fall or winter, possibly due to the greater amount or intensity of physical activity in the summer. This study examined seasonal differences in injury incidence during US Army Basic Combat Training, where physical activity was similar at all times of the year. METHODS Four independent groups of subjects (total n = 1543 men and 1025 women) were investigated, two training in the summer and two training in the fall. Injury data were obtained from a retrospective review of the subjects' medical records at the conclusion of the 8 week training program. RESULTS For men, the corrected relative risk of suffering an injury or a time loss injury in the summer was, respectively, 2.0 [95% confidence interval (CI) = 1.7-2.4] and 2.5 (95% CI = 1.9-3.0) times higher than in the fall. For women, the corrected relative risk of suffering an injury or time-loss injury in the summer was, respectively, 1.4 (95% CI = 1.3-1.6) and 1.7 (95% CI = 1.5-2.0) times higher than in the fall. These risks were essentially unchanged after adjustment for subject physical characteristics (age, stature, body mass, body mass index) and physical fitness (push-ups, sit-ups and 2 mile run), indicating that the summer season was an injury risk factor independent of these variables. Linear correlations (r-values) between maximal daily temperature and injury incidence ranged from 0.92 to 0.97, indicating a strong relationship between these two variables. CONCLUSIONS These data suggest that injury incidence among physically active individuals is higher in the summer than in the fall and that environmental temperature may provide a partial explanation for this finding.