Objective: To determine the rate of injury in junior Australian football, and to describe the patterns and severity of these injuries across nine levels of play (U9 to U18). Design: Prospective cohort study. Setting: Junior Australian football games and training sessions were observed for 54 teams from New South Wales and Victoria over the 2004 playing season. Participants: Six teams from each level of play were invited to participate in the study. Overall, data were collected for 51 teams over 40 208 hours of player exposure. Independent variables: Participation and injury data were collected prospectively. Main outcome measures: Injury was defined as "any trauma that causes some disability or pain''. Injury severity was identified by the action of players immediately after the injury event. Results: The overall injury rate was 18.0 (95% CI 16.6 to 19.3) injuries per 1000 player hours. The main cause of injury was body contact (67.3%). There was an increased frequency of sprains and strains, and injury severity with increasing level of play. The rates of injury for players who stayed off the field (6.4 injuries per 1000 hours, 95% CI 5.6 to 7.2) or were advised to seek off-field medical advice (5.0 injuries per 1000 hours, 95% CI 4.3 to 5.7) were low. Conclusion: Compared with the adult game, junior Australian football is relatively safe. However, injury rates increase as children progress across age-determined levels of play towards the more adult form of the game.