Although injuries to the upper extremity are most costly, the picture of the upper extremity injury problem remains incomplete. This study is the first to describe the etiology and mechanisms of upper extremity injuries in Danish children. A 2.5-year observational prospective cohort study was conducted to record upper extremity injuries in 1048 children. Data were weekly collected by sending a text message. A total of 176 upper extremity injuries were reported (128 acute injuries). Of the acute upper extremity injuries, 55% were sprains, 47% occurred in the hand/wrist, and 53% of cases were caused by a fall. When corrected for exposure to physical activity, this resulted in an acute upper extremity injury incidence density of 0.18 per 1000 h of physical activity. The odds of sustaining an upper extremity injury was higher in the older children (HR: 1.84, 95% CI: 1.10-3.09), a tendency was found suggesting that girls are at increased acute upper extremity risk compared to boys (HR: 1.40 95% CI: 0.97-2.04). The findings that most injuries occur after a fall, that injury risk increases over age and that girls seem to be at increased injury risk provides essential information to guide future childhood injury prevention.