Abstract Background Mild traumatic brain injury (MTBI) is a leading cause of injury for children during their pre-school years. However, there is little information regarding the long-term outcomes of these injuries. Method We used fully prospective data from an epidemiological study of a birth cohort to examine behavioural effects associated with MTBI during the pre-school years. Cases of confirmed MTBI were divided into two groups, those that had received outpatient medical attention, and those that had been admitted to hospital for a brief period of observation (inpatient cases). The remainder of the cohort served as a reference control group. Results Mother/teacher ratings for behaviours associated with attention deficit/hyperactivity disorder and oppositional defiant/conduct disorder, obtained yearly from age 7 to 13, revealed evidence of deficits after inpatient MTBI (n = 21), relative to more minor outpatient injury MTBI (n = 55) and the reference control group (n = 852). For the inpatient group there was evidence of increasing deficits over years 7-13. Conclusions More severe pre-school MTBI may be associated with persistent negative effects in terms of psychosocial development. The vulnerability of pre-school children to MTBI signals a pressing need to identify high-risk cases that may benefit from monitoring and early intervention.