This article examines the evidence that the effect of head injury on young children may differ from that in adults, in that while in the latter the pattern is of deficits that recover with time since the accident, this is not necessarily the case with very young children. In this group, there may be no evidence of any deficit in the early days or weeks after injury, but the children may fail to develop some skills as quickly as children who have not had a head injury. Results from a series of studies of MHI in preschool children carried out over a more than 10-year period from Auckland Hospital and recently published studies of pediatric MHI from other groups are reviewed. It is concluded from a comparison of these data that there is a need for long-term prospective studies designed within a developmental framework to clarify the issue.