The criteria for hospital admission of children who have suffered a minor head injury are highly subjective. Often the presence of post-traumatic emesis becomes an influential factor, but the mechanisms that trigger emesis following minor head injuries are not known. From a prospective study of 96 consecutive children with their first mild head injury (GCS 13-15) and a retrospective study of 29 consecutive more seriously injured children (GCS 8-12), we conclude that post-traumatic emesis is more common: (1) following minor head injuries than following more severe head injuries (P less than 0.05); (2) in children over 2 years old; (P less than 0.001); (3) in children injured within an hour of a meal or snack (p less than 0.001). The presence of a skull fracture or the site of the impact does not influence the incidence or duration of post-traumatic emesis. Retching and vomiting generally subside within 3 h in children injured within an hour of a meal or snack. When vomiting appears in children injured more than an hour after a meal or a snack, it may be quite protracted (mean = 7.5 h). Children over 2 years of age with post-traumatic emesis who are neurologically stable following a mild head injury that occurred within an hour of a meal or snack can be expected to improve quickly. Their counterparts injured more than an hour after a meal or snack are likely to remain distressed much longer and are best admitted to hospital.